A couple of threads on AtariAge are exploring the possibility of using the "unstable" opcodes in this group (ARR, etc.) as a sort of fingerprint. The hope is that the instability is a prediction of the specific model of CPU. To what end I'm not sure of yet, but it's interesting research all the same.

https://forums.atariage.com/topic/385516-fingerprinting-6502... https://forums.atariage.com/topic/385521-fingerprinting-6502...