Duncan doesn’t necessarily have a stash. They come with the molded covers, already installed. They may send/sell a cover but it’s extremely risky to replace. They’re glued on, and bending the circuit board can easily and instantly break one of the 4 coil wires going down to the pins.
If you do get inserts, they’re not that easy to replace, you’d have to heat up the old ones until they slipped out, and then probably use heat, then glue, to make sure the new ones go in and stay there permanently.