I had one of those 2/50mm jobs; wish I'd kept it.
If the lens won't come off, try relocating it fully in place and run the aperture ring up and down a few times; there may be slack in the movement that's stopping it from coming off properly. That little forked tab used to connect with a pin and operate the metering if you used a metering pentaprism, if my memory is still working correctly. That's why the lens had to be attached with the ring at the recommended aperture, which I think was wide open. That allowed it to mate correctly. In fact, now that I think about it, I still fit a lens on my digital bodies only with the aperture wide open.
After use, I always open the aperture to the max again, both because it makes later swaps easy, but also because I remember reading that it weakens the mechanism if you leave the lens in stopped-down mode after use.
I think the same applied to the Hasselblad 500 Series bodiies too: always fitted lenses with the aperture wide open. How strange how easily one forgets things that were done a thousand times without a conscious thought! Or it might just be me.
Rob