I don't often do a midweek #DavesCarIDService but this cool photo request sparked my interest because it prompted a double and triple take, and a lot of deep digging to figure out what this thing was.
First, I can tell you that the bike here is a circa 1955 Cabton (either a 500cc or 600cc RTS), and despite the quasi-British sounding name, it is thoroughly Japanese.
Note the British appearance as well, looking much like a Brit twin of the era, rather than the odd duck pressed steel frames of cheaper, smaller Japanese bikes like Honda. Cabton (from Mizuho Mfr Co) was among the first Japanese motorcycle companies to emulate that style.
Cabton were large displacement by Japanese standards, 350cc-600cc, with vertical twin OHV motors based loosely on Indian "Dyna Torque" and Arials. These were only made 1954-58, but company made spare parts until 1960 before finally going under.
The "Cabton" name is actually an acronym, for Come And Buy To Osaka Nakagawa."
I can also tell you a first year 1954 Cabton RL can briefly be seen in the original Japanese language version of Godzilla (1954) 
There aren't many contemporary advertising or promotional images of Cabtons at the time, but they were definitely cool and promoted the image of Cabton as a bad boy's bike.
The story of Mizuho/Cabton ended tragically in 1960, when financial problems prompted company president Naito to commit suicide by wading into a river near the factory.
And now for the kicker: I don't know if this is good news or bad, but if your dad held onto that bike, it has serious collector value. They are extremely rare, even in Japan, and have a bit of history as a speedway racer. I'd check if Dad has it stashed somewhere in the garage.
