Fuel tanks and locking caps

Ok, the only information I have on my fuel tanks is a number 128D10. I am looking for locking gas caps for my tanks. Both are 120 gallon tanks, and the fuel fill area is recessed and there are no threads to where the cap would screw in the tank. Just the ears on the cap where you twist to secure it, and the rubber is all that is holding them in place. I do believe they are original, as the date of manufacture is 1978.

Thanks,
Vampire

 

Its a 78 Kenworth W900A. There are no threads for the cap to screw into. It is just a smooth surface on the filler neck. The current cap has ears on the cap, when you turn it closed the ears lock into the tank. The filler neck does not protrude from the tank. I'm going to call Robond and ask them about it tomorrow. Thanks Hanadarko.