Kevlar PASGT US Military Helmets

I have been trying my best to find out what the dash numbers (-3,-5, etc) mean after the sizes. Inside each helmet there is a size imprinted from the mold such as M-4. I know the M means Medium, what does the -4 mean? Measuring helmets from M-1 thru M-6, i can find do dimensional correlation.

Could the dash number equate to a mold shell number?