fnfalman
Contributor
The M1's second stage has a heat exchanger to keep it from getting freeflowed when water temperature drops down low. Also, the design of the purge cover helps attenuate purging if encountered high water flow such as swimming into heavy current or in caves where water could be channeled through small openings at high velocity. Another interesting feature for the M1 2nd stage & octo is that they could be fitted with "cave" rings where it allows the regulators to be disassembled underwater for quick cleaning. Sometimes heavy silt encountered in cave or wreck diving could jam up second stages.