Odiwan72 got me looking at this (thanks a lot, Markus!).
I found a couple things that may or may not be legit. They are close either way:
Or to be closer to what I'm thinking:
This is is an Eaton Aeroquip Coupling Assembly for an Airbus. Probably not a copy of anything old enough to be the receiver, but possibly same size (or close) as an Airbus is huge. Unfortunately, the listing this is from did not have dimensions listed. I'd say that it looks very similar to the furthest rear section of the receiver (not including whatever the red thing in the first pic is).
Then there is this, which I don't think is THE part, but very very close. It is listed as an MG42 tripod mount. Again, no dimensions given.
I have some Priority Mail boxes at home. I will try to match up which size box this is, and then try to get a rough estimate on the dimensions for this piece. Obviously some portion of this would have been removed for the Jawa blaster. Also I believe it would look closer to the real Jawa blaster if the "trigger" was swung around against the other side that is shown in these images (if that is possible). However, the rivet area holding the trigger to the rest of this assembly is not shaped narrow enough in my opinion.
Finally, I have been searching WWII era heavy machine gun pintles, but the general design/engineering theory behind the Jawa blaster's trigger housing "arm" appears to me to be more like a small automobile transmission yoke (or maybe half of a CV joint), like from say, a late 70's/early 80's Volvo maybe? Or perhaps from some other small or old vehicle. I suspect the rear portion of that entire "arm" shape is actually the transmission yoke and then some other part attached to that belt drive gear (in the front), and they are held together by that blackish tube in the middle of the whole arm piece. These are just wild guesses though.