I have a 527M carbine dated "14" (which I purchased new) and the used carbine I picked up last Thursday is a 527 (no "M"), it is dated "04".
My "M" is a youth carbine but from reading, the youth model is a stock change only. Side by side the only difference I can see (I have not removed the stocks) is the shape of the trigger. One is more straight/flat and the other is curved. Other than that the barreled actions, bolts, trigger guard, sights all appear identical. One of them does have a thicker butt pad (The non-M if I recall correctly....they are at home I am at work, can't check that out right now, I also think the M is the one with the straight trigger).
I have searched for this on the Net but found nothing...I'm sure someone here knows.
TIA