Is it 9 or 18 volt? If it’s 9 it’s most likely an MIM, if it’s 18 it’s most likely an MIA. There is also no active/passive toggle which could be either COO depending on year of manufacture. I can’t recall off the top of my head, but the MIA Deluxe got an A/P toggle sometime in the 2000s and the MIM deluxe got one a few years later. IDK if the MIM went to 18V when it got the A/P toggle, but It should say on the headstock somewhere what the COO is.
Either way, unless you can find a used top knob your best best is to just buy the set of two for that pot. Fender also sold the stacked pot knobs separately, but they have never been very plentiful. I don’t think it’s anything but the (relatively) small number of active basses sold that usually makes the stacked knob set an order only part for most suppliers. They can get them, but very few stock them.
Last, the stock pre consists of 2 separate part numbers, one is the board with 2 pots mounted to it and a third attached with wire so it can reach it’s hole position. The second part number is for the remaining pot and must be connected to the board. I don’t remember which part number contains the stacked pot, but I do know that none of the knobs come with either part number, but are generally available as a full set. They are easier to find as a full set, which may be why it’s tough to find the stacked set alone.