For those not familiar with these old IBM machines that used decrementing of the index registers, this will help explain all the ??D instructions are there.

https://bitsavers.org/pdf/ibm/704/24-6661-2_704_Manual_1955....

Another lens for the FORTRAN case specifically, in the modern way we tend to say Fortran arrays are 'one' indexed, looking at this old code it is sometimes useful of thinking of it as a limit index, pulling from n-1 -> n. Or at least I found that useful for myself when digging into some of this a long time ago.

thanks for finding that, i love it!