A number of these rifles came into the US as unissued surplus in the 1990s/early 2000s. Fazakerly made them until at least 1956 for various contracts - Ireland, Malaysia, Royal Air Force, etc. Seems most seen unissued in the US fall in RAF serial number ranges. Yours is probably an RAF gun, given its serial number. I can't find the chart at the moment, but off the top of my head, I am pretty sure it is RAF. They came to be known as "Irish Contract" Enfields, though the Irish serial number range is relatively small compared to total production and it is questionable that Ireland ever took delivery of their rifles.
There are various theories as to why certain rifles came in unissued, many in the mummy wrap in which they were placed when they were built. In short, though, the world was moving over to semi-automatic and select-fire battle rifles and the Enfields were of a bygone era that was rapidly passing away. There were no major conflicts in the mid 1950s in which these were needed by groups like the RAF, so they sat unissued until surplussed decades later.
Very nice find. They are fantastic rifles and shoot really well.