I just happen to have the PRR hopper book open nearby.
The PRR H31 classes built during WWII, similar to the MT 2 bay hopper, still had 632 of them on the books in April, 1968.
I'm surprised by the numbers of the ancient GLa class, built before WWI, that were in active service: 3,410 in 1965, 231 in 1970, and 77 in 1973.