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..
[1] https://www.washingtonexaminer.com/tech-timebomb-the-irs-is-...
[2] https://www.bloomberg.com/view/articles/2018-04-17/the-irs-c...
The first COBOL compiler was available 2 years later in 1960. Algol 58 wasn't available until 1960 either.
The ALGO (Algol 58) manual for the Bendix: http://www.piercefuller.com/collect/bendix/algo6008.pdf
The first compiler for PL/1 was delivered much later, in 1966.
You could have suggested LISP 1.0 but that appeared in 1960 too (as an interpreter, the first compiler was written in 1962).
LISP, COBOL, ALGOL: 1960 was a remarkable year in the history of programming languages.
The system ran on a mainframe not 50 but 60 years ago. The only high level language at the time was FORTRAN, but this system was apparently written in assembly language.
Does anyone know the type of computer it ran on? The article it linked to says IBM mainframe but which model?