Are they interchangeable with butt plates on the soviet hard wood stocks?
Nope, they are two distinct buttplates. You can make one fit and vice versa, but they have distinctly different inletting of the buttstock so you could get a gap that might look a bit ratty in some instances when putting a laminate buttplate on a hardwood stock.
Something like this:
(https://preview.ibb.co/ib3x7e/P756_buttplate_bottom.jpg) (https://ibb.co/cLgTDK)