In the Bell System world the backboard for the modular wall sets is the 191C-XX. It snaps over the 630A4 modular jack for decor purposes. Mounts 550, early 1550, or 2550 series modular wall set, or AC1 base (Trimline™). It was available in standard WECO colors. It replaced the 182B and 191B backboards.
It could be used with Stromberg-Carlson and ITT modular wall sets too. Although , there was another company, CAC, I believe that made them also, but they have been gone a long time. There may have been some others also, but I did not find any in my old North Supply and Allen Tel catalogues.
You might try E-Bay, is it surprising what shows up there at times. Just type in WESTERN ELECTRIC and you may be amazed.
....JIM....