Music Masters was based in Eastcote, Middlesex. It appears to have gone in for albums rather than singles or EPs, but a few of the latter were issued; the example shown dates from 1979. All of the Music Masters records I've seen have been Regimental or Brass Band music, which suggests that the company leaned heavily in that direction. Both sizes of record were numbered in the same MM-0500 series, which eventually became MM-0600; that said, the prefix appears to have been omitted in the case of the EPs. The highest number that googling reveals is MM-0654. The records are undated, but the company seems to have been in operation from the late '70s to the mid '80s.
Copyright 2006 Robert Lyons.