sean...The gas cap is of the sort used on the majority of British Two Stroke bikes. The wording around the edge are the brand names of the oils we remember from a long time ago. The cap has a cylindrical "cup" underneath, "the measure" which the rider filled with his favorite lube and added to the gas, in the proportion stamped on the cap. The gallon refers to the imperial quantity. That it fits the tank is a more of a coincidence.....
Swarfy.