Hi Joe
I just looked at photos of both arms. The TP90 arm photos all showed conventional looking cartridge mounts on what looked like standard plug-in type headshells. The TP16 arm turned out to be the arm I recently installed a cartridge in. I think you have the TP16. You have to screw upward into the sliding metal shim-looking plate. It has threaded mounting holes but you have to have screws of just the right length for your cartridge. My problem was that in my case the little plate had corroded and you couldn't hardly see the corrosion filled mounting holes. I was able to make it work, kind of.
Good Luck,
Bill