There were only 4 3-carriage sets of the 64' multi-doors, so seeing them on around the BR(S) region would have been rare.
To answer the question of the frames, yes, I believe they were Maunsell frames from before WWII, and were used up by Bulleid after the war. However, as said they weren't in a great state even then, so later in their lives they deteriorated more quickly than other new builds.