I have PBR's and pre 64's.
I preferred the old trigger mechanism, it was the best over ride trigger on the market but according to the articles I read, tolerances were such that a very light trigger pull could not be made with parts assembled on a production line.
Still it was a world wide proven trigger.
The new actions are a bit safer with a gas blocking shroud on the left receiver rail. Parts are not interchangeable, complete bolts will sort of fit, but I did not file to fit, bolt shrouds won't. Threads are different. Can't use the new M70's for some critical parts on the old.
The new action is machined better than any M70 to date. My PBR's are slick.
I liked the fact on the old pre 64's feed lips were machined into the receiver wells. As long as they are properly milled you did not have to worry about messed up magazine boxes causing mis feeds.
I don't like the detachable magazine on my PBR's. The edges are sharp, magazines three round only, easy to push a cartridge out the back when loading. Plus you cut your finger doing that.
I am of the opinion that the FN M70's are going to be the best made of all M70's, which when you count in the pre war M70's, that is a very high standard. Post war 64's were hit and miss in quality throughout the entire post war production. A good one as a good one, a bad one was like finding a turd floating in the punch bowl. And there were enough bad ones to show that clowns were in charge of manufacturing at New Haven. The action was always a great action when properly made. The rifle is still a great rifle. M70's, good ones, are still one of my favorite rifles.