Looks like you've found the culprit Tom. The fact that you didn't notice this on other negs may be due to the fact that the marks "lined up" with higher density portions of those negatives. I wonder if the black marks are residual paint from the negative carrier. The scratches probably wouldn't be a big problem if they didn't have density. Perhaps that could be cleaned out. Alternatively you should be able to replace the diffusion material. I don't know if Meopta sells this part - and even if they don't, some other neutral diffusion material should work. The alternative, of course, is to take only photographs with highlights in those exact areas. Guess not!
Seeing the Meopta takes me back. I used to sell Meopta enlargers at my Dad's shop back in the sixties and seventies in Surrey. ( Aahh - the "Good Old Days" - back when men were men and wore floral shirts and bell-bottomed jeans )
Post how it works out Tom. Good luck mate.