It seems to me that the Joint Army Navy (JAN) equipment SYSTEM designation does not always square up with with the Unit that is part of a system.  The R-. T- or RT- numbers assigned to the major components seem to be assigned much later in the design process.  Is the SYSTEM number assigned when the contract is awarded?   But the R-, T- and RT- along with any control (C-) or power supply (DY- or PP-) UNIT numbers are assigned much later after the prototype meets the preliminary spec?  Is there a method to SYSTEM and UNIT number assignments or is it just kinda willy-nilly?  

Then there are the gaps in the SYSTEM number lists by Andreas Parsch.  Are these gaps the result of systems that never made it to production?  Case in point is the AN/ARC-26.  I found reference that this was assigned to a Collins Radio Navy contract in 1947 which was then cancelled in 1952.  Andreas lists it as a communication set for the AC-130 which didn't exist until the mid 1960's, AFAIK.

Jim