The Savage parts finder will only bring up factory replacement p/n's for your gun as it left the factory. Savage could care less if you want to convert your gun or change something, that's your mess to figure out. To get the metal bottom p/n you'd need to enter the serial number for a gun that originally came with one.

As for the Numrich's models, look at any of them with a C in the model designation. It's kind of hard to say because over the years there have been different DBM setups and all have used the "C" designation for a detachable magazine model, so make sure you've got the right setup before committing. Numrich's schematic's aren't always helpful either so don't rely on them to be 100% accurate to that specific model.