There was a recent thread that mentioned Bushmaster currently gets their lowers from CMT, and used to get them from LMT. The prefix letters of the serial numbers apparently denote where a Bushmaster lower receiver came from.
Just so you know, when you purchase a CMT lower, you will not necessarily be receiving an exact duplicate of the ones produced for the big companies, which all have their own specifications of how they want their receivers. For example, RRA lowers exhibit dimensions that are different from the norm. If you purchase a lower from the manufacturer that produces lowers for RRA, there is no guarantee that you will get the same characteristic tightness.