The Marines SOCOM units were using G19’s obtained from SOCOM those were Glock 19 Gen3s and some Glock 19 Gen3 MOS models (not commercially availible). SOCOM subsequently began buying the G19 Gen4 MOS as the Mk27.

The USMC subsequently bought G19M’s off the FBI contract for plainclothes CID and HMX 1 (the presidential helicopter squadron). Then they bought some more 19Ms as an “interim” for an AFG deployment pending evaluation of the M17/18.