Its not surprising. a lot of years things can happen.
My guess is at some point the LH studs were replaced with aftermarket RH studs, ("no marking on the drivers side rear") aftermarket studs typically do not have the R since it is implied that all studs are right hand and the front drivers side was changed too but the PO put the OEM hub\drum on the drivers side. with all right hand studs they are interchangeable.