> The only high level language at the time was FORTRAN, but this system was apparently written in assembly language. No direct insight to the actual system use

> The only high level language at the time was FORTRAN, but this system was apparently written in assembly language. No direct insight to the actual system used, but FWIW IBM mainframe assembly IMHO seems 'surprisingly' user friendly due to the large amount of standard macros/routines provided, clear/direct integration with JCL, and the minimalist ethos of the OS. Take a look at some youtube videos for some examples, it actually looks like a fairly interesting/productive, if arcane, environment. Writing clean/well architected applications in it doesn't actually look like it would necessarily be a ridiculous notion. Of course, how well any given production system was constructed and what cruft has accumulated in it over 50y is probably another story..