Spectrum Volume/Tone knobs

Are the knobs for the Spectrum LX/DX/MX series the same for both guitar and bass instruments?

Yup.
Proprietary, and rare as rocking horse doo-doo if you're trying to replace 'em.  

And just for fun there are metal and plastic versions, as well as some with metal inserts and set screws, and others which are friction fit. Lots of experimentation over the various instrument model year releases, but same design.

Yeah... Barry has it right! They do pop up from time to time on sleazebay, or someone gets a bee in their bonnet and makes some from scratch - I copped some in red and in blue from this forum which now grace an MX and a GT....