I don't think that tank is from an earlier bike or a truck. I have seen too many single fill, late style caps to think they all came from discarded trucks. My theory is that they may have been factory replacement tanks. The part number for a tank is 629, whether early or late style. When they changed to the new style cap, I believe they discontinued the early style tank and just sold you a new style one. Now, if one were buying a replacement tank for an earlier bike, they would want a single fill tank. This was available as part number 629T, which was built for the trucks.
Now, as to why his bike may have a single fill tank, when it probably had a 2 fill tank when new? The owner went to his Servi-cycle dealer for a new tank, and that is what the dealer had on the shelf!
That's My Story, and I'm sticking to it!! LOL!!
Another thing, they made single fill late tanks in both left and right fill!
Rick