Jennings Electronic Industries O50 (Organ 50)
Serial number 306
Probably produced in early 1970. The latest observable date code (on one of the RCA driver transistors) is "9K" = November 1969. The inspection stamps on chassis and mains transformer need closer inspection.
Very few changes to the original electronics, perhaps the only one being the Daly filter cap on the driver board in the "well" of the chassis.
In common with the Jennings B50, these amps have sockets for "preamp signal out", and signal into the power amp.
For pictures of a slightly later JEI 050, see this page, where notes on the circuit are being assembled.