I am under the impression (maybe incorrectly) that V3R7 was an intrim release. only used to migrate from a CISC box to a RISC box.
we operated v3r5 and one of the procedutes of the CISC/RISC upgrade was to upgrade to v3r7 (on a CISC) as an intrim step to v4r2 (on a RISC).
in another AS400 group i mentioned v3r7 and everybody got upset and said there was no such operating system. i proved it to them via the IBM CISC to RISC book and page and they stopped talking to me. so i left the group.
so you may have on your hands a CISC box that has been upgraded. there may not be a v3r7 cum package. see if you can find the CISC to RISC pub.
but, if there is a cum package, IBM has what’s called ‘gold’ tapes. they may have not kept them due to the age of the equipment. it’s like looking for a program for Windows 3.1 or DOS 3.