I must be the only person baffled by this story. First, I've bought dozens of parts from Rock Auto for modern cars and the Chevys and have never had a bad experience. But more importantly, according to my 1938-1964 original Chevrolet dealers parts catalog and according to my original '64 4-door hardtop I checked tonight, the wheel cylinders ARE THE SAME LEFT AND RIGHT REAR. The line on the driver's side one goes in angled from the back, the line on the pass side one goes in angled from the front. They're the SAME PART, Part no 5459918 in the parts book! If you think about it, why wouldn't they be? There's no logical reason for them to be different. By the way, they cost $5.60 at your Chevy dealer in 1964.