I have, what looks to be a mid/late 60's Gazelle kwikstep. The tires are weathered beyond positive identification. I measure the BSD at approximately 19.2" (488mm). According to Sheldon's work, it seems to land somewhere between the 451 and 507 (the two most common US sizes). The bike is in very nice, original condition so I'd prefer not to deface it by swapping wheels. Any help who be greatly appreciated.