Sounds like you have a Redi-Mag on the lower of a R6601. R6601s had serial numbers in the "MH" range. I don't think there were any in the "MN" range.
If I'm right, your gun was probably made in the early nineties, not the late eighties. Also, it should have an A2 stock, A2 sights, and a 20" 1/7 Hbar barrel with no bayonet lug.
Let me know if I'm close.