Something that doesn't seem to have been mentioned previously is that the 1960s decade was a period of flux in this country and the Empire/Commonwealth countries. Although India parted in 1947, it was Ghana in 1957 that really set the ball rolling, with most countries gaining independence in the next 10-15 years, which coincided with new currencies and therefore issues. As many countries used the Royal Mint to supply their new coinage, blanks for these issues could be a possible source of many off-metal strikes. A lot of currencies are nominally similar in diameter and weight to our own currency, so as long as we are talking about regular metals - bronze, Cu-Ni etc, there is a distinct possibility that they were genuine mint errors.