I know a 600cc "UOR" engine was listed as available for HRD's in 1926, but if any were acturally built and sold is beyond me and there were no HRD's in 1930, but it is still possible a Special Vincent-HRD could have been built with a UOR Engine in 1930. Is it a Twin Port or Single Port and can you post a picture of the engine and it's numbers?
Cheers, John