Note
Radio, Dec 1940, p. 69.
QST, Jun 1947, p. 143.
This is the same device as the CPO-124, but without a speaker. It can be attached to the CPO-123 (dynamic) optional matching speaker. Note that the CPO-123 is visually identical to the CPO-122 (magnetic) speaker.
Brief review. Radio, Jan 1941, p. 134.