First, just to get the numbers clear...the 7408 is the 14K SOLID gold Dress watch. If you have two of those, you're doing well. Otherwise, the 5408 is the goldfilled version. I have a couple magnets, and a minty goldfilled Auto-Set case back (no magnet recesses) I can sell.
There was no provision on either model for storing a magnet in the clasp. Instead, the 102-equipped models (both solid and goldfilled) came with a P-shaped magnet that was able to be carried on your keyring (one edge was even serrated to more closely resemble a key...this is NOT to be confused with the "P" magnet for the P1).