Thought it would be good to demonstrate the variation in fuel caps as we haven't covered it in any great detail. Fuel cap designs varied considerably over the life of the Model 4 Automatic, Model 4 Special and Model 5 Special. The image below shows four known variations that were adopted over life of those models.
Beginning with the top left, this version was used on the Automatic and as with the front Predicta flap, is a made of a clear/white plastic, painted with Hammertone. This example came from an Automatic with a serial number in the 117,000s.
Top right we have the version that is most often confused with the "true" Automatic fuel cap. It differs most significantly in that it is made from non-transparent green plastic, and is not painted. This example came off a Model 4 Special with a serial number in the 154,000's.
Bottom left is the version adopted early in the life of the Model 5 Series 1. Black unpainted plastic, with the Victa logo. This example came off an early run M5S1 with a serial number in the 189,000's.
Bottom right is the plain black fuel cap used for the majority of the Model 5's existence. The precise point at which this final state was adopted is not yet clear, but this example came from a Model 5 Series 2 in the 290,000s.
People have been known to try and pass of the second version as the true Automatic version from time to time. The easiest way to differentiate between the two is simply look at the back of the cap, as seen in images 2 and 3.
If I've gotten anything wrong, let me know fellas!