Ya just google "Johnson serial number search" or something like that. Serial number is on the side of the mount. It is probably J8xxxxx. (my Johnson 7.5 says 8 too…)
The letters correspond to a leg length, starting style (elec or pull), year, and sub model (if applicable.
Having the info really helps for getting parts at the store, as well as searching for yourself online when the parts guys say that they don't have it (but you know they do!)