Dear Mr SixpackSuperBee - this is the .280 British calibre prototype as examined by the US government in the early 1950's. I have never seen one. Ever. And I was a frequent visitor and user of the MoD facilities in the Pattern Room, where all YOUR Sharps prototypes are permanently held, as well as about 200,000 other weapons.
After the war, and in response to the need to have a smaller more easily handled infantry rilfe with enough power ot do the job, the British came out with EM2, a radical bull-pup design. The Belgians came out with the FAL, and in an attempt to pursuade the allies to buy it, made trial units in .280 British to present to us and the US, as the latter at that time did not have an intermediate cartridge [the .30cal M1 carbine did not count as an full-blown infantry weapon]. The US, who by that time was developing the round that became called the .308Win/7.62x51NATO] were not interested. The British governent canned the wonderful bull-pup EM2, which we should still be using today, and bought the FAL which we, along with Canada and the rest of the commonwealth, built to imperial standards rather than metric. We called it the SLR L1A1 [Self loading rifle], and it served us all well from 1956 to an undisclosed time that is not as far back in the past as you imagine. I loved it. It was accurate up to 600m or so, worked no matter what you did to it, and fired a serious round that really smarted, althoguh on a good day you would not have felt a thing. Unlike the Daisy-thing we have now, I never saw anybody take a 7.62mm hit and get up and walk away.
The US trial weapons are lost to history, but your weapon is a rarity indeed. It's value cannot be estimated if both the MoD Pattern Room and FN are interested in seeing it again.
By itself it is an interesting piece of British and Belgian history, a might-have-been. It is certainly far rarer than any Borchardt could ever be.
Give me a call on pm if you want any more info.
tac