Further research in my books has nailed this down to being in fact a bolt-guide from a
Gewehr 43 ( Kar. 43 ). The G.41 has a machined bolt-guide, the 43 had a die-stamped one ( which this relic certainly is ). The real give-away is the finned alloy cover, a very clear picture of which is in Smith's manual.
http://www.militaryzone.cz/Pics_Clanky/G43/Gewehr43.jpg
I'm really very pleased with this 'find'. A relatively unusual weapon for Normandy and overall I'd rate it as the 'star find' of my weeks' holiday in the Falaise Gap.....
