Ok this might not matter as you stated yours might have been modified.
Anyway back in the early years of the Rocket there was actually two oil tanks in which you ordered by engine number. Anyway then along came the update kit. which had components for the oil tank and fittings
There were two update kits one for the silver engine and one for the black. But I think the only difference was the color of the out put shaft cover. You might want to inquire so you can tell you bought the right parts for the tank. Meaning if you have the old components are you getting the right O-rings ?