Gas tank question

[JMRide76]

Hey guys,

Just bought a 69 k10 swb a few weeks ago and trying to get it running. It had a locking gas cap on it and the previous owner had lost the key. I got the cap off and I have bought two caps from local parts stores and they were both too big and would not fit.

I was checking out the LMC catalog and I found that there are tanks with and without vent hoses(evaporative emission control). Mine has a vent hose. But according to their catalog, those tanks are only on 70-71 trucks.

This truck is not original and I don't have a lot of information on its past. I do know it was partially restored when the previous owner bought it. Is it likely that whoever did the previous restoration put a 70-71 cab on a 69 frame? Is there anyway I can confirm this? The front end is a 69-70 front end. The plate with the VIN number is not mounted to the cab, it is laying in the glove box.

Also, is the fuel cap smaller on the tanks with the vent hoses? Thanks for any help.

John