NOTE: OPF-P365X, RMRcc is incompatible with P365 Macro, or the new (October, 2022) version of P365X that has the rear sight in its own dovetail, as shown below:
It is compatible with the older generation of P365X, where the optics plate and rear sight are integrated:
Given's SIG's penchant for changing things with abandon and no apparent reasons, here's another thing to look out for.
Please remove the slide's optics cut cover plate, and ensure the slot is present. OPF-P365X, RMRcc has an oval shaped indexer that mates with the slot on the slide, but some P365X slides do not have this feature.
Your slide must have the slot in the center as shown below: