An eBay auction references a 4.75" Cuda bolt circle. Is this merely a mistake on the seller's part?

Most likely. 64-69 Cudas will have the small 5 on 4 bolt pattern unless someone has modified the axles/hubs by swapping parts or redrilling. I believe that the 70-up E-body Cudas had the larger 5 on 4.5 bolt pattern. Moparear wheel drive cars and trucks only used bolt patterns of 5 on 4, 5 on 4.5 (by far the most common) or 5 on 5.5 (trucks and vans post 85). I am not aware of Mopar eveyr offering a factory 4.75 bolt pattern, especially on a Cuda. maybe if the brakes were swapped to some kind of aftermarket setup that used Chevy parts? I think Chevy had some 5 on 4.75 bolt pattern vehicles.

Thanks. I think maybe Chevy is the only thing to have 4.75 stock.

Most RWD GM vehicles used the 5x4.75" pattern, except the really big cars which used a 5x5" pattern.