[Milsurplus] Pre-WW2 USAAF nomenclature ?

[Hue Miller]

I understand that in this example, SCR-AQ-183, BC-AQ-430, the letter Q
is the "production series" -or  is there a better way to describe this?
Contract?  But what about the letter A before the Q, what does it mean?

What does the first letter of this two letter group mean? How about in
this example:  SCR-SA-167  ( radio compass receiver system ).

Has anyone ever seen the BC-183 external CW oscillator for the 
SCR-183 ?

Apparently this style of noting the successive productions was
abandoned about the time of the BC-224, 1936, or we would
have BC-AA-224 and not BC-224-A.
-Huey Miller