"SJ" and "SD" radar - what does it stand for?

[Nisgeis]

Originally Posted by LukeFF

Any number following the two-letter prefix denoted a factory modification. A lower-case letter denoted a field modification. In reference to WWII radar, SJ-a was a field-modified SJ radar set modified with a PPI screen, and likewise SD-a was an improved version of the original SD (what is known as "improved SD radar" in SH4).

I typed out the thing about field mods and then deleted it, because I can't remember the source I got that from and I'm not actually sure I ever did see one. Do you happen to know of a source for that field mod convention?