Unfortunately, I don't have any breadcrumbs for R15's old value. However, I was able use the offsets in the RP's parameter list to work backwards to its original value.
Now it all flanges up except for my understanding where the 0C4 came from. The instruction that's the target of the resume is referring to key 8 storage which matches the PSW's program key. The base register address in the instruction is valid for both 24 and 31-bit addressing. The PSW is in 31-bit addressing mode. Thanks for everyone's replies.
