I recently had a Traxxas 3.3 with the Opti drive given to me with transmission all disassembled. It has the grey transmission and receiver box. The trans is taken apart and I'd like the forward only conversion in it but HobbyTown sold me the wrong part it seems because I can't figure this thing out. They gave me a 5394x part which looks like it could be for the revo. Also went to another hobby shop and got 4994x this is confusing... could someone please get me going into the right direction.