Mamiya Pro SD film back lever

Hi everyone,and hello from sunny London
I hope someone can tell me what the small black lever on the top centre of the film back is for????

It is called a film wind-stop lever according to the manual. It releases, as one would expect, the film wind lever, should that be necessary. On very rare occurrences the wind (advance) lever can get temporarily stuck in the advance position. The small metal lever on the center top, which you asked about, frees the advance lever to return to the rest or home position.

Thanks for that !