I'm wondering what these two pieces look like coupled. The AP M4E1 threaded should be a regular forged 5.56 dimension upper that's different on the outside, so there should be absolutely no compatibility issues with the FM-9. IIRC, the Aero 9mm upper has a BHO that would interfere with the FM-9 lower's BHO.  I'm referring to Foxtrot Mike's FM-9; apparently there's more than one company using that model designation.

Thanks for any and all helpful info and a safe weekend to all!