It is the turbine modifier that was originally attributed to "MC" (due to insufficient and vague information).

-The engine in question is the General Electric T-58, a turboshaft for helicopters built between 1955 and 1984. There were made 6300 engines of this type.

“Les Shockley T-50”

-This engine was named "Shockwave 800+". The power turbine -transmission to the rotor- was eliminated and a central cone and an outlet nozzle were placed, as we can see ostensibly in the photo.