Ah, that looks like the takedown Greener GP shotgun action - and a splendid one for a medium-pressure rifle it is. Off-topic this may be, but you and others here may be interested in the following source of parts, for the standard military Martini:
http://www.martinihenry.org/index.php?route=common/home
I recently bought a barrel from them which should restore an early-sporterised Martini to a more meaningful state of life.