have never seen this set-up prior to your photos - and a quick look at the indexes of the crudgington & baker books has shown no light.

i assume this is spring loaded inside the forearm frame, and would seem to promote a constant pressure against the action pin, rather than depending upon the usual bearing on the forearm against the knuckle - looks useful and clever to me.

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland