There was no such thing as an END-arith. I assume that the NOT alternate clause was an IBM extension. I am not sure of that. For clarity it is important to understand that this early compiler went through release stages, the best was Realease 2. At this point I should say that old code never dies we just keep regressing new compilers.
Current managers enjoy gifting conversion efforts to the next generation.
The earliest version of this did not support NOT alternatives. That is actually a cleansing phase. If you wanted to use this unparameterized, you NOT clauses died. That bought time to convert your code. You could actually use the new compiler and run time, and figure out how you finance conversion later.