Answer for COTS parts

Yes, “A” would be equivalent to “M”.

The failure rate designation is signified by a single letter (M, P, R, S), which identifies the failure rate level for which the resistor is qualified (at 50 percent rated wattage).

Failure-rate level Failure rate
designation percent/1,000 hours
——————————————-

M……….. 1.0
P……….. 0.1
R……….. 0.01
S……….. 0.001