Sorry but I’m completely lost. Why the mention of Glock OEM parts for use in a Sig Sauer P365?
You are lost, but it's a complex issue.
The mention of Glock OEM parts was to make note of the possible differences between OEM parts and after-market parts.
The firms that made metal frames for Glocks encountered a problem with after-market parts when installed in the new metal-framed Glocks: some parts providers were NOT consistent in their production of parts, and the specs varied. These variances apparently didn't matter when installed in polymer-framed Glocks, but often would NOT run in metal-framed versions. (The same part ordered from a given supplier could have different specs, if ordered again, several weeks later.)
I mentioned it BECAUSE there's a chance that the same sort of issue may arise with after-market parts for a SIG running an after-market metal frame...
Particularly when the after-market parts are used to do something the OEM part doesn't do -- like shorten the reset or trigger pull. (When the part is no longer made to match OEM dimensions/specifications, what is correct?) And if there's later subtle differences with a later production run, who is to say the original was right and the later versions aren't -- if they work in the polymer-framed guns, but not the metal-framed versions?)
I've often thought an aluminum-framed Glock would be an interesting experiment, but after reading about the problems with Glock metal frames, I knew that there may be more problems than I wanted to deal with to get one that really worked as it should.
Most of the after-market parts and enhancements were NOT developed with metal frames in mind, and I suspect that there was little incentive for those suppliers to make changes for a very small potential customer base.