The etymology of ingot is not absolutely clear; Etymonline echoes the OED with its concise “probably from in- ‘in’ + O.E. goten, pp. of geotan ‘to pour.’” But it seems to be a native Germanic form, and the -t has always been pronounced… except that Spenser writes (The Faerie Queene 2.7.5):
And round about him lay on euery side
Great heapes of gold, that neuer could be spent:
Of which some were rude owre, not purifide
Of Mulcibers deuouring element;
Some others were new driuen, and distent
Into great Ingoes, and to wedges square…
Some editions have “Ingowes” rather than “Ingoes,” but only heavily modernized nineteenth-century versions have the modern spelling. Could it be that he was borrowing a French version? The French is lingot, with an initial article having melded into the word (itself borrowed into Spanish as lingote), but perhaps he got hold of the pre-melded form. Or maybe he just didn’t know how everyone else pronounced it.