Many of you said that this was a BSA A7, and while you were all incredibly close, that wasn’t the exact answer. Instead, I have to congratulate reader dnaj who correctly identified this as a Meguro K1, also known as the Stamina. Impressive work! Meguro apparently bought an example of the BSA A7 in 1953 and then built their own clones (many riders thought the clones were better bikes than the originals).
As mentioned in the original post, this is reader Cas V’s bike – so I’ll let him share a little bit more about his project:
“1961-ish Meguro Stamina, a 33hp, 500cc twin. This was an improved version of the BSA A7. Even the BSA engineers said it was what the A7 should have been. Although the engine looks identical, its not a direct copy. Better bearings, better machining – all bolts etc in metric. The carb is stamped “Amal Japan”. This one was a police model. I’m guessing surplus after the Tokyo Olympics. I have no idea how it found it’s way to Hawaii as they were never exported. A couple years later (1965), the Stamina was beefed up to 650cc and called the Meguro Model X with 47HP. Kawasaki took full control of Meguro at this point and exported the Model X as the Kawasaki W1. The rest is history.”