No.5 Jungle Carbines only have 4 numbers, the Shirley carbines have BB to C? prefixes, last production was post World War 2. The Fazakerley jungle carbines ran from FE1 to FE1000 initial production, then with no letter prefix, followed by A1 to A9999 through Z9999.
G6XX - ?/45 - Y - Anubis Priest ( Your date cannot be 1948 for a FAZ G prefix = 45 )
Could it be a spare receiver used to replace a damaged receiver, and the original # was used?
I like Ganderite's answer, but could conceivably be an FTR that they did not write FTR on.
The seller ad claimed it's a "rare, 1 of only 200 factory No. 5" rifles made in '48.
But I've never seen documentation to support production went past '47.
Dunno. there were a bunch of other pictures in the ad, but it looks like the ad has vanished.



























