Surprise!!! 

Such a pair of ingots was shown as cargo on a boat on a Mohenjo-daro tablet. m1429 Prism tablet with Indus inscriptions on 3 sides. Since Indus Script writing is most pronounced in the Mature phase of Sarasvati Civilization, the tablet can be dated to between ca. 2500 -1900 BCE. It is surmised that the ingots shown on the boat were produced by Meluhha merchants because the language used in the script is Meluhha rebur renderings of metalwork.

Slide 24. harappa.com Moulded tablet, Mohenjo-daro.Three sided molded tablet. One side shows a flat bottomed boat with a central hut that has leafy fronds at the top of two poles. Two birds sit on the deck and a large double rudder extends from the rear of the boat. On the second side is a snout nosed gharial with a fish in its mouth. The third side has eight symbols of the Indus script.

Material: terra cotta.Dimensions: 4.6 cm length, 1.2 x 1.5 cm width Mohenjo-daro, MD 602.Islamabad Museum, NMP 1384.Dales 1965a: 147, 1968: 39

The pair of ox-hide ingots which flank doorways on Bharhut scultpural friezes also occur on an Indus Script inscription on Mohenjo-daro prism tablet m1429. The two ox-hide ingots are shown as cargo on a boat flanked by two palm trees and twwo auatic birds.

Bharhut sculptural frieze showing ox-hide-shape ingots flanking both sides of the doorway. Elephant signifies karibha, ibha 'elephant' rebus: karba, ib 'iron'.

Hieroglyph: కారండవము [kāraṇḍavamu] n. A sort of duck. కారండవము [ kāraṇḍavamu ] kāraṇḍavamu. [Skt.] n. A sort of duck. कारंडव [kāraṇḍava ] m S A drake or sort of duck. कारंडवी f S The female. karandava [ kârandava ] m. kind of duck. कारण्ड a sort of duck R. vii , 31 , 21 கரண்டம் karaṇṭam, n. Rebus: karaḍā ‘hard alloy’ (Marathi) karaṇḍa ‘duck’ (Sanskrit) karaṛa ‘a very large aquatic bird’ (Sindhi) Rebus: करडा [karaḍā] Hard from alloy--iron, silver &c. (Marathi)A pair of birds కారండవము [kāraṇḍavamu] n. A sort of duck. కారండవము[ kāraṇḍavamu ] kāraṇḍavamu. [Skt.] n. A sort of duck. कारंडव [kāraṇḍava ] m S A drake or sort of duck. कारंडवी f S The female. karandava [ kârandava ] m. kind of duck. कारण्ड a sort of duck R. vii , 31 , 21 கரண்டம் karaṇṭam, n. Rebus: karaḍa 'hard alloy (metal)'. tamar ‘palm’ (Hebrew) Rebus: tam(b)ra ‘copper’ (Santali) dula ‘pair’ Rebus: dul ‘cast metal’ (Santali) 

Ox-hide ingot is so-called because of its shape https://www.andywhiteanthropology.com/blog/the-oxhide-ingot-from-lake-gogebic-michigan " of Turkey (source in text). For those of you unfamiliar with the term, an oxhide ingot is an ingot of copper cast into a quadrilateral shape with concave sides and four "handles" (apparently its called an “oxhide” because it resembles a stretched animal hide in shape).  During the Bronze Age in the Mediterranean (ca. 3200-1000 BC), copper was smelted and cast into ingots of this shape weighing about 60-70 pounds (~30 kg) for transport. "

Source of tin in AFE powering the Tin-Bronze Revolution of 4th millennium BCS.https://tinyurl.com/y5rbl2ap

The provenance analyses by Begemanna et al is conclusive evidence for the source of copper which powered the Tin-Bronze Revolution in ANE. The stunning finding is that the copper came from Khetri mines, not far from Rakhigarhi. One reason why Rakhigarhi was the capital, the key river port town of the maritime riverint-waterway civilization. Similar provenance studies should continue to firmly anchor the source of tin. The largest source was in AFE. See map. 

Enormous quantities were used to progress the revolution. What is the largest tin belt of the globe? It is in AFE. I have posited this for further detailed analyses and testing. (So have Muhly and Potts,, archaeometallurgists).Tin ore is accumulated as placer deposits by Himalayan rivers grinding down granite rocks creating cassiterite ore.

See cylinder seal showing tin and copper merchants from Meluhha. 

https://tinyurl.com/y2lpc55b

Shu-ilishu cylinder seal with Indus Script hieroglyphs, Akkadian cuneiform inscription confirms Meluhha trade in copper and tin (Hieroglyphs on the hands of Meluhha merchants signity that they are traders in copper and tin: mlekh 'goat' rebus: milakkhu 'copper'; ranku 'liquid measure' rebus: ranku 'tin ore')

Begemann, Friedrich & SCHMITT-STRECKER, S. (2009). Über das frühe Kupfer Mesopotamiens. Iranica Antiqua. 44. 1-45. 10.2143/IA.44.0.2034374. A lead isotope study »On the Early copper of Mesopotamia« reports on copper-base artefacts ranging in age from the 4 th millennium BC (Uruk period) to the Akkadian at the end of the 3 rd millennium BC. Arguments are presented that, in the (tin)bronzes, the lead associated with the tin used for alloying did not contribute to the total in any detectable way. Hence, the lead isotopy traces the copper and cannot address the problem of the provenance of tin. The data suggest as possible source region of the copper a variety of ore occurrences in Anatolia, Iran, Oman, Palestine and, rather unexpectedly (by us), from India. During the earliest period the isotopic signature of ores from Central and North Anatolia is dominant; during the next millennium this region loses its importance and is hardly present any more at all. Instead, southeast Anatolia, central Iran, Oman, Feinan-Timna in the rift valley between Dead Sea and Red Sea, and sources in the Caucasus are now potential suppliers of the copper. Generally, an unambiguous assignment of an artefact to any of the ores is not possible because the isotopic fingerprints of ore occurrences are not unique. In our suite of samples bronze objects become important during ED III (middle of the 3 rd millennium BC) but they never make up more than 50% of the total. They are distinguished in their lead isotopy by very high 206 Pb-normalized abundance ratios. As source of such copper we suggest Gujarat/ Southern Rajasthan which, on general grounds, has been proposed before to have been the most important supplier of copper in Ancient India. We propose this Indian copper to have been arsenic-poor and to be the urudu-luh-ha variety which is one of the two sorts of purified copper mentioned in contemporaneous written texts from Mesopotamia to have been in circulation there concurrently.

The shape of he boat on the moulded tablet is comparable to the Bronze Age Uluburn ship which had a shipwreck.I suggest that this boat was in charge of a supercargo (rebus: karNi Most frequently-occurring hieroglyph on Indus writing corpora: 'rim-of-jar') of copper and tin ingots, based on a rebus reading of the hieroglyphs on three sides of the prism tablet, including a text in Indus writing, apart from the ligatured hieroglyph of a crocodile catching a fish in its jaws [which is read ayakara 'blacksmith'; cf. khar 'blacksmith' (Kashmiri); 

khār 1 खार् । लोहकारः m. (sg. abl. khāra 1 खार; the pl. dat. of this word is khāran 1 खारन्, which is to be distinguished from khāran 2, q.v., s.v.), a blacksmith, an iron worker (cf. bandūka-khār, p. 111b, l. 46; K.Pr. 46; H. xi, 17); a farrier (El.) Side C:

Thus, the indus script inscription of m1429 Mohenjo-daro seal is a bill of lading listing details of cargo loaded on the boat:read rebus:
alloy metal smith, ḍhālako ‘large ingots’, hard alloys 

karaḍa 'hard alloy (metal)'. tamar ‘palm’ (Hebrew) Rebus: tam(b)ra ‘copper’ (Santali) dula ‘pair’ Rebus: dul ‘cast metal’ 

ingots (of smithy), alloy metal (of smithy), lapidary work of stones, minerals, gemstones, (metal) equipment, (authorised by) helmsman.