Looking at their lists of duties, it seems MR's are machinists, and MM's repair machines/pipes, etc.
MR What they do The duties performed by MRs include: ? repairing and operating auxiliary machinery; ? calculating time and material needed for machine shop work and the repairing of auxiliary machinery; ? drawing sketches and preparing specifications for replacement parts; ? working from sketches and specifications to produce replacement parts; ? operating machine shop equipment, such as lathes, drill presses, shapers, bench grinders, milling machines, boring mills, power hacksaws and computer numerically controlled machinery; ? working with precision measuring instruments such as micrometers, depth gauges, verniers, calipers, gauge blocks, protractors and dial indicators; ? engraving metal and other materials; ? performing electroplating and flame spraying operations. --------------------------------------... MM What They Do The duties performed by MMs include: ? aligning piping systems for oil, water, air and steam, and controlling the operation of ship boilers and steam turbines used for ship propulsion and service systems; ? controlling operation of turbo generators used to produce electrical power; ? cleaning, adjusting, testing and performing other preventive maintenance on a ship's boilers, main engines, turbo generators and other auxiliary machinery including steering engines, elevators, winches, pumps and associated valves; ? operating and maintaining desalinization plants (distilling plants) to make fresh water from sea water; ? maintaining refrigeration plants, air conditioning systems and galley equipment; ? repairing or replacing valves, pumps, heat exchangers, compressors, steam turbines and hydraulic or pneumatic control devices; ? making entries in and analyzing machinery operating records and reports. --------------------------------------... You should be able to see the difference in primary duties.
