Don't need an M in the type at all, many springs don't yet have a meters per second rating. Literally, any spring between 90 and 190 will be m/s rating, so take the number and multiply by 3.3 and you'll get a rough idea.
Also, there are springs that have a % rating, first of all they are largely flaky (made by PDI), same rating in the past has been different lengths and strengths, I've had PDI 150% springs that ranged from 6 3/8" to 7" long, made of different thicknesses of wire, and put out anywhere between 350fps and 425fps.
Add in, different manufacturers will have springs of the same ratings, but different outputs for some reason (likely test rig differences). I used to have a Prometheus 110 spring that had a constant output of 360fps in my MP5, yet a Madbull 110 would net me 415fps.