You can see the Zone system dial on your insert. This is for the 205 spot metering camera.
You can see the lack of contacts on the face of the shell. This shell is for 500 series non electronic cameras.
Hopefully the vendor has not sold the back that has the 205 shell with the 500 insert that is sitting in his shelf!
And hopefully you can get the two apart without damage.
I don't think I will to simulate the mismatch with my gear, but I wouldn't bet the lockup is the mismatch. My gear looks identical in all relevant ways inside.
Be sure the lip on the slide is the right way and all the way in. the release knob needs to be worked, or wiggled or turned one way or the other, to get the insert out.