Well, Your high cab looks to not have the original tank. around mid 36 the side of the fuel tank had a filler neck. and the 34/35 trucks had a cap like what you have in the picture but, more in the center. and the tank is a littl diffrent in design where the stock 36 tank looked just like the 34/35 with just the cap hole not cut. and with the filler neck on the passanger side that comes out the side.
It looks to be a replacment tank and where they installed that cap set up on the top and looks like they used tar to seal it. that is my guess as to what the black stuff is.
Poets Seat AutoAppraisals, Parts, ServicePast: General Motor's Master Technician
Member:
Northeast Chevy/GMC Truck ClubPhone Number: 413.774.5856
Collector of: 1951 Army Chevy Burb barn door,53 GMC COE,two 1934 Chevy Truck,1935 Chevy Army Truck.1950 Gmc Suburban clamshell these are just my chevy/gmc trucks.
