According to the 1952 AN/FRR-24 manual, the 125 cps (Z-401) and 250 cps
(Z-402) filters were manufactured by Vectron Inc., 235 High St., Waltham MA
Z-401 was a crystal filter and Z-402 an LC filter. The FRR-24 was initially shipped without the crystal filter which was provided later. Unfortunately I can't read the date on the Z-401 filter box that I have.
|FRR-24 filter assembly as delivered
||As shown in the manual
|Crystal filter provided later