Aftermarket. Probably an old Confer tank. Instructions in my sig line. They are quite nice IMO considering the mileage these gas hogs get but they can rust and are kinda thin. I just had one lined and it was not cheap but the added capacity in the truck I am building will be welcome.
Handy and timely post. the '65 I just bought has the same, though the tank is long gone. Still has the lines etc. so probably would not take much to hook one up. Note sure I feel like spending 1G on it though!
Bought my 68 back in 1974, been in AZ the whole. Has the same gas door as this 69. Itis either or not auxiliary fill main tank. The lever to switch tanks is on the floor between the toolbox and door. I have a 70 with a different opening but the same tank. The valve on the one is at the back of the transmission cover under the passenger's seat. Both tanks have skid plate the hangs off the frame tube around the rear axle and rear cross member. I have a few other spare tanks that hang off rear floor and just a round hole for the gas cap. Being this 69 is AZ curious how close the spare tank is to my 68.