I've been looking at CONCERT-DISC releases, and hope someone can provide some information.

There appear to be 2 series of single LP's and a series of Box sets.

A popular series, lowest number I've seen is CS-20 (stereo), mono equivalents M-1020 ff.
  What is last (CS-53) ??

Classical series CS-201 (stereo), mono equivalents M-1201 ff.  
  What is last (CS 261) ??

Box sets in a 500 series SP-501, mono MP-1501 ff.
  What is last (507) ??

Were the box sets a product of the original company, or solely from Everest ??

There are a number of series designations
  Sound in the Round
  - what is the significance of these, does AUDIOPHILE have anything to do with the Audiophile label ??

Best wishes, Thomas.