What is the quality like for these lenses? This says the system was discontinued in the 1970s.

https://en.wikipedia.org/wiki/Mamiya_Press

I've seen a lot of Mamiya 645 and 67 systems but those were probably from the 1980's and 90's.

I can't speak for the Mamiya Press but the Zeiss Planar 80mm f/2.8 for the Graflex XL is rad.