Standard Tanks 74-79 have the cgas cap in the center of the tank. All Special Tanks have the cap to the side.
Left; 74/75..Center 76-79. Right Special 78-83
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74/75 Standard tanks have a raised cap and no side badges., No aftermarket cap alternative. If a tank for sale doesn't have the cap it is worth considering passing up on the deal. Used or OEM gas caps are very hard to find or very expensive if you do.
76/77 Standard tanks have a similar gas cap to the specials. and no side badges. Aftermarket Caps available
These standard caps look the same as Special caps but they arn't and do not interchange
78/79 Standard tanks have the same gas cap as the 76/77. These 2 years have badges on the side
There is a difference with the petcocks.
74-77 have manual petcocks
78/79 have Vacuum operated petcocks
78/79 petcocks are wider at the base and taller than the early 74-77 petcocks. Note in the pics, (silver tank and dark tank with badge missing), the base of the tank where they sit, you can see the earlier Tanks protrude where the petcocks sit. Later Vacuum petcocks tanks are slightly recessed to accommodate the extra height in the petcocks.
Early and late tank petcocks do not interchange. If someone has filled in the bade on the later 78/79 tank check the petcock mounting to determine what tank it is.