To finish this... After serial No. 17,691, the DC-24 in a FT-241-A case was used (mostly). The 5-pin to octal adaptor 2Z262 was required when this later crystal was used. Info from TM 11-275, 24 January 1944. A standard 100kc rock would probably work fine. Dennis D. W7QHO Glendale, CA