Help! - Interesting Crank, but what is?

[odthing]

Does anyone know what this crank might be from. The bore is about 62.8mm, and the stroke around 90mm (hard to measure), suggesting about 500-550cc overall. OHV flat top pistons and the flywheel has been nicely polished at some time. Note the big end caps are fixed with studs rather than bolts, and the crank is three part. The crank casting is marked Qualcast. All info welcome.