Rare? Why? This coin is not even uncirculated. Post if you know why.
Strictly a guess. Wrong metal or wrong die axis?
Well, I don't know why, but I am going to hazard a guess. 1946 was the last year for British coins to have 0.500 silver; in 1947 they went to CuNi. Is it possible this coin is a CuNi that was used for testing, and somehow go out into circulation?
Wow, you are so spot on!!! This coin was struck in copper nickel and was in all likelihood a pattern for the upcoming coins that were to be struck in copper nickel for the 1947 year. I actually got this many years ago in a Belgian auction of all places and not seen ANY others before or since; supposedly such coins were struck also in the sixpence and florin series but nobody I have ever talked to has seen them...
Funny how even though I collect the series and therefore have my "eyes up" for various coins coming up that I just happened to come across this coin at auction - even then (12 years ago about) it cost 800 euros plus commission. I have tried to track down the other denominations to no avail.
Pretty cool find!