This submission popping back up from the second chance pool got me to do some digging for the formal description of system/360[0], this is not the APL we know today but the APL outlined in Iverson's A Programming Language[1].

[0] https://www.cs.trinity.edu/~jhowland/class.files.cs2321.html...

[1] https://www.jsoftware.com/papers/APL.htm