I used to work for a company running a COBOL mainframe on a Tandem system, which was upgraded from a variety of older computing systems, and contains code to emulate the behavior of a Burroughs Mechanical Calculator so that certain tabulating processes can continue to work.
If MOCAS is the oldest program, then the compiler they use must be younger. Have different compilers been used over the years, as the hardware changed?
A potential spring chicken, relatively, in the oldest compiler stakes:
Around 1990 while I was in college a friend of mine and I met with a guy that ran a very small credit union in town that was looking to replace the IBM mechanical accounting machine they were still using to keep records (something like this - but it looked a little more modern like it was built in the 1960s). We ended up not working together with him but it was neat to see that machine in action.