KS number etched in the glass
refers to The Kearney Specification (named for WE's plant at Kearney, NJ) was the medium to set requirements for material bought by WE for distribution to the Bell companies. There were KSs for everything under the sun - oil for
teletype machines, transistorized amplifier-equalizers for radio program lines (KS-20159), resistors (an ordinary half-watt carbon resistor was described as KS-13490 List 1), quartz crystals (KS-14193, equaling military CR-18/U), and so on. The various sizes of Hammarlund APC variable capacitors of '40s vintage were covered by 26 various KSs.
There was some elegant McIntosh tube amplifier covered by a KS, intended as a central-office distribution amp for wired-music (Muzak) networks.
In a lot of cases (like the KS-20159 amp above), the device was actually a
Bell Labs design, and might include a lot of WE-made parts, and was
documented in Bell System Practices prepared by WE, but an outside
contractor did the manufacture.
Many thanks to my pal Ludwell Sibley
for the above information.