Not sure how the Non-CVW helos and the FTLSTP communities do it, but if follows some of the same logic
Wow, you guys are WAY more organized than the USMC CH-46E world...
For us, each different squadron has it's own MODEX. I think at one time it probably made sense, but it's not as logical now. Here's the breakdown:
MCAS New River, NC:
HMM-162: YS (now a VMM, bastards...)
HMM-261: EM
HMM-263: EG (now a VMM)
HMM-264: EH
HMM-266: ES (now a VMM)
HMM-365: YM
MCAS Camp Pendleton, CA:
HMMT-164: YT
HMM-268: YQ
HMM-364: PF
MCAS Miramar, CA:
HMM-161: YR
HMM-163: YP
HMM-165: YW
HMM-166: YX
MCAS Futenma, Okinawa:
HMM-262: ET
HMM-265: EP
If I was a betting man, my guess is that it used to be 1XX for a West Coast Squadron (combined with a "Y" MODEX), 2XX for East Coast (and "E" MODEX), 3XX for Hawaii (No idea of MODEX, but I'd guess "P"?). When the Phrogs left Hawaii, one squadron went East, and one West (hence each coast has one 3XX squadron). Then, when they went to Okinawa they grabbed two squadrons from the East Coast and backfilled with one 1XX squadron from the west coast. I could be WAY wrong, but that's just a guess. As you can see, it's mighty confusing.