It's a home made box fitted with a Microwave Modules downconverter (42MHz crystal so 70MHz-42Mhz=28MHz) with the IF frequency available on a Belling Lee socket to provide the LO for the transmit converter. I bought and built the Spectrum Communications transmit converter as I had previously built a similar unit for 6m as it was released for amateur use around then. (1984/5). Although no longer listed on their website Spectrum Comms are still in business and quite probably have the documentation available. From memory, it was around 0.5 Watt output and that would match nicely with the PA unit, something I acquired at a junk sale as likely being suitable. The PA device is a 2N5643 rated at 60W DC and 40W RF with a gain of 8dB so you should be able to get a few watts out of it. Its probably off the back of a FM PMR set. I've run the input coax into a MFJ antenna analyser and there is a dip around 55MHz but trimmers are there for adjustment to suit. Its all 'as seen' and a starting point that might get someone going on the band, even just RX to start with.