The 40/50/60 are thread sizes based on a weight measurement; the higher the number, the finer the thread.
I have never seen MM on thread spools, unless possibly you mean mm as in millimeter? The amount of thread on a spool or cone is typically given in either yards or meters, but meters is a single m.
Probably all of the thread I buy has the weight, ply and length on it. For example, Aurifil 50/2 is 50-weight 2-ply (my favorite for both piecing and quilting on my domestic machine). Standard sewing thread that I grew up with is 50/3. It may be that this is such a standard thread size now that some companies do not bother to label it that way. Maybe?
Last edited by Prism99; 11-07-2012 at 09:03 PM.