Looks like a "Baillie-Grohman peep sight". There's one on the M/S M1910 I traded into, but mine is not engraved as yours is. Also the profile at the forward end is different than mine.

There's a few good pictures at the bottom of this thread.

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=211610&an=0&page=0