[Milsurplus] SCR question: SCR-AA-187 (1934) Versus SCR-187-A (1936)

[Mike Morrow]

Pre-WWII Signal Corps Type Numbers were in two different forms as shown in this comparison:

SCR-AA-187 (1934) versus SCR-187-A (1936)

The 1940 General Catalog of Signal Corps Equipment states:

"Suffix letters are used in type numbers to differentiate between articles having a general similarity, and do not indicate that previously assigned similar type numbers are obsolete or have been superceded.  In lieu of a suffix letter, a second combination of letters before the numerals of the type number has been used in some instances in the past to indicate a particular procurement or design of an article manufactured under a performance specification."

Pre-WWII U.S. Army equipment nomenclature can be confusing, with unfamiliar type numbers assigned in odd sequences and combinations.   The best examples of that are the first six SCR-Ax-183 sets, listed here according to order date:

Date         Maker   System        Receiver     Transmitter
06/25/1932   A.R.C.  SCR-AA-183    BC-AA-179    BC-AA-180
03/15/1934   A.R.C.  SCR-AC-183    BC-AB-199    BC-AB-200
03/15/1934   Graybar SCR-AD-183    BC-AD-199    BC-AD-200
04/19/1934   Graybar SCR-AB-183    BC-AD-219    BC-AD-200
06/05/1935   A.R.C.  SCR-AE-183    BC-AE-229    BC-AE-230
07/27/1935   Graybar SCR-AF-183    BC-AF-229    BC-AF-230
             for WECo

SCR-AE-183 became the pattern used for all following sets except SCR-AF-183.  Later sets often had mixtures of earlier components such as SCR-AH-183 items that were also part of SCR-AJ-183 and SCR-AK-183.  The -AJ- set has no individual components labeled with -AJ-.  The last -AS- set had one component labeled -AT-.  There are many other quirks.  The study of the taxonomy of these sets can be the basis for a minor hobby activity...I know!

There would eventually be 21 different SCR-Ax-183 (12vdc) and SCR-Ax-283 (24vdc) command sets.  

Nine other sets use SCR-Ax-### nomenclature:

Date  Model        Service            Receiver      Transmitter
1932  SCR-AA-192   Liaison Receiver   BC-AA-179
1934  SCR-AD-192   Liaison Receiver   BC-AD-199
1934  SCR-AA-185   Observation        BC-AD-219     BC-AA-191
1934  SCR-AA-187   Liaison            BC-AD-229     BC-AA-191
1935  SCR-AB-185   Observation        BC-AF-229     BC-AA-191
1935  SCR-AA-186   Radio Compass      BC-AE-229
1935  SCR-AB-186   Radio Compass      BC-AF-229
1935  SCR-AB-187   Liaison            BC-AE-229     BC-AA-191
1936  SCR-AC-187   Liaison            BC-AD-229     BC-AA-191     

In 1936, the SCR-xy-### form was abandoned:
1936  SCR-187-A    Liaison            BC-224-A      BC-191-A

The same form of nomenclature was used for many individual components of some systems.  For example, system RC-12 is an external oscillator used with SCR-Ax-183-type receivers in liaison service to allow CW/A1 reception.  Its main component was one of six oscillator units:
BC-GL-183, BC-GM-183, BC-RA-183,
BC-RB-183, BC-RC-183, BC-RD-183.

If a component with this type of nomenclature is modified, it may have a suffix letter.  For example, SCR-AF-183 had some items modified to improve sidetone that then became:
BC-AF-230-A   Transmitter
BC-AF-231-A   Receiver Control Box
TM-AF-172-A   Junction Box

SCR-xv-### format was first used for SCR-AA-183 in 1932.  The last example is SCR-AS-183 in 1942.  Other than the legacy use for SCR-Ax-183 and -283, the last use of the SCR-xy-### form was SCR-AC-187 in 1936.  ALL such sets were aircraft-mounted.  This agrees with the earlier 1940 quote saying this form was "used in some instances in the past".

Mike / KK5F

One great SCR nomenclature mystery remains:  Why do many write "SCR-274N" instead "SCR-274-N", as it appears in all official military usage?  :-)