I've found the same thing in both my '50 and '54, i.e., the tank
was apparently installed before they sprayed the interior.
Both a Personally I wouldn't worry about the paint sticking
to the tank, heck, the original paint stuck to the original
tanks for 50, 60 or 70 years and taint flaked off yet.
I've also found on mine and several others that the filler spout
was painted body color: http://www.pbase.com/dennygraham/1950_cab&page=4
The interior and blue were the factory paint job, confirmed by
the original owners who I got the truck from.
Denny Graham
Sandwich, IL

Denny G
Sandwich, IL