Note
QST, Mar 1970. p. 105.
CQ, Mar 1970, p. 113.
QST, Aug 1971, p. 117.
Ham Radio, Feb 1972, p. 81.
Ham Radio, Mar 1972, p. 78. (PR-550A)
Probably introduced in 1969, with the GT-550.
The PR-550(A) is a hybrid phone patch with and recorder control circuitry. Panel jack accepts S260 (PL68) plug. Selector switch (lock or momentary) controls PTT line. Function switch selects desired audio routing for recorder or play-back to equipment or phone line .
Gain controls on receive and transmit.