Never heard of such a thing. If that's indeed what it is, it looks to be a field expedient modification.
Never heard of such an item, & yes, it does look a bit 'home made'..........or should I say, a 'field assembly'.
Homemade is one of the questions I had - the grip is simillar to to a front grip of the Thompson SMG.
The other: How does it work?
Turn our ot was a Canadianprototype winter trigger. Source documentation was ID'd in this thread Anybody ever saw this? | Gunboards Forums