>>2863057 prev bread
Taking a stab at this, although I like the decryp of "DECLAS", too. Looked at it from standpoint of each letter-number combo as its own acronym.
FWIW, I think it's useful if everybody goes at these crumbs from a bunch of directions. Greatest chance of finding correct solution.
C-3: C3 – Command, Control & Communication (Human activity focus)
A-5: https://en.wikipedia.org/wiki/North_American_A-5_Vigilante
S-6: https://skarnet.org/software/s6/
D-1: https://en.wikipedia.org/wiki/Richter%E2%80%93Veresov_Attack
E-2: https://www.h1base.com/visa/work/Compare%20work%20Visas/ref/1133
L-4: https://l4.ventures/