I was looking into it after I posted a question earlier, and found this very thorough description of the Platinum Preppy line of products. Much better presentation compared to the Platinum website or the Platinum US distributor website (which has information about a different product line on the preppy page).
Not mentioned here, but you can use an adaptor to use an international style cartridge in your preppy fountain pen. It works lika a charm.
Edited. I wrote converter, but I meant to say cartridge
If I convert, I’ll probably used that o-ring gasket method.