Old thread, but I thought I might add some information that I came across while researching the "MX" serial number prefix for myself.
A local Winchester collector says the "MX" prefix indicates a new serial number being assigned to a rifle by Winchester, if the original serial number was unreadable when the rifle came in for rebarreling or extensive repair. Winchester would remove the original serial number, assign it a new number, and add the MX prefix.
This, apparently, was only used for rifles getting rebarreled, and/or receiving substantial repair and refinishing work. And the serial number had to be unreadable upon arrival.
So, it's a no-go from a collector's standpoint, but not really a detractor for the average gun lover.
....Hope that helps someone.