By 1930 the use of valve stem covers which were usually used on tubes with metal valve stems was phasing out. Plymouth never used them in 1930 if the fact that they are not listed in the Plymouth parts catalogue can be taken as evidence. A rubber stem tube with a cap was all they used. My car has dual side mounts and one of them was original to the car according to the original owner I bought the car from in 1962 and that's what it had. By the way, the term "schrader valve" just refers to the valve that is screwed into the valve stem. That said, there's no reason why you can't consider the valve stem covers you have to be an after market accessory and use them if you like the way they look. Personally, while I don't have them on my 30U, I like the way they look and they certainly dress things up a bit.
