The MKIII was made W/E 14/11/1964. Part of batch 5 of 6 batches made that week. Info just says .22". I understand as well as the X or O prefix smoothbores also had a proof mark; unless that was for a specific market, but conversations with the B P H confirm. ITS THE BLACK POWDER MARK FOR 1926 ????? The stock looks factory made to me but it has no inlet disc which is a bit odd; if its a copy it's damn good. Hope the info helps.