From looking at the parts breakdown, it don't appear that the 555 DTC and 665DTC have the same transmission, case, gears and etc. Maybe someone has more knowledge about these tractors than I do. I did look at the 665DTC pretty close and I don't see an inspection cover on the side of the rear end case. However, I do see a round cover on the 555DTC rear end picture.
Farmtrac 665DTC, Kubota L2550, John Deere 310SE, Dresser 175C
Maybe it is different enough that I won't have the problem. I did check the parts manual breakdown and I see the pinion bolts, doesn't appear to have any lock on it. When I'm back over at the farm I'll take a look at the inspection cover.