SCM SCR SCS

Just when you think you have all your jargon and technical terms down someone whips out an acronym you don't recognize.  The acronym SCM stands for Set, Complete, Meteorological. It's one of three related WWI Radio Signal Corps acronyms. More here.

• SCM - Set, Complete, Meteorological
• SCR - Set, Complete, Radio
• SCS - Set, Complete, System

 The book World War II Battlefield Communications by Gordon L. Rottman covers this in some further detail. U.S. military radio equipment has been designated S.C. since 1909 standing for Set Complete.This may have distinguished it from radio parts in that era. This was a prefix to every model number. For example an SCR-300 was a portable radio transceiver radio used in WWII. The SCM-1 was a Mobile Meteorological Station used in the same conflict.In that context by the way mobile means vehicle mounted. More here.

There was also a huge number of two-letter designators for the components and accessories for all these units. Some were pretty obvious, AT indicated antennas, HS for headsets, LS for loudspeakers, GN indicated generators, BA indicated batteries. BC was less obvious,  basic components like a receiver. But RD stood for switchboards and EE for telephones, 

In 1943 still in the middle of WWI the military introduced an all new numbering schema. This was called the "Joint Electronics Type Designation System" aka the “Army-Navy System” or “AN System.” (It was not actually used by the navy.)  It was later changed to JETDAS with the inexplicable addition of the word "automated."  But thankfully they did not re-designate existing models. As time passed the new system was applied to new devices and as technology progressed those older units fell out of service.