The "M" series as you refer to them are all US and (Canadian) military trailers. This group included a whole bunch of different trailers. Many pretty specialized (water tanks, generator sets, rolling tool boxes...) The best known are the general cargo trailer which include the M100, M416, M101, M101cdn and the M105.
These cargo trailers range from 1/4 ton capacity (off road) on up to 1.5 ton payload (again, this is the off road rating).
They were made by a variety manufacturers. Just depended on who got the contract. All built to the same specifications for the model, no matter who built them.
None available new, all surplus. Most now in private hands (some of these date back to WWII).
Mark...