Chris was correct in that Motorola was the originator; NMO as in New Motorola. Unfortunately, they are not all the same. One of the best attributes of the original 3/4" NMO is that it is waterproof even when the antenna is removed. The smaller 3/8" and 5/8" ones are not.
You might think this wouldn't be a worry with a mag mount, but it is. Rain water mixed with dirt and road grime will corrode the contacts with obvious results. This occurs when there is a mismatch between manufacturers.
Lastly, I don't like mag mounts for safety reasons. In the event of a crash, they come off and become flying missiles which can maim and kill. Don't laugh as there has already been one reported death caused by one. By the way, most insurance companies will not insure any device not permanently mounted to a vehicle. Caveat Emptor.