Beautiful part! Is this the styrene master or the cast?
I wondered on those parts too! Though your part is not present on the Pheasant the fixture that is attached to the breech handle, and engages this mystery part, is (see photo below).
Your mystery part has a ramp that engages the cam and roller attached to the breech handle.
Since it only works one way (there is a stop on one side of the roller and on the handle) it will possibly be a mechanism that automatically opens the breech, and ejects the shell casing after comming in battery again after recoil (the example in the photo has not returned into battery, it got stuck halfway recoil).
It could also possibly automatically pretension the spring to open/close the breechblock, so it operates with less effort. Fascinating stuff, hope someone knows the answer!
Marcus