I think that it is impossible to tell what is early and late. I think they made both types of filler caps simultaneously. I am pretty sure, though, that the ones with the four bungs for petcocks in the rear are the earliest. They had 8th inch bungs for smaller early BSA petcocks. The later ones had 1/4 inch bungs that stuck straight out the back. Many of these are stamped "Made In Japan" on them somewhere, but even the Japanese ones were made as early as 1968 or so.
the earliest dated one I have seen is on a Triumph Drag Bike ridden by Bobby Sirkegian in Pomona in 1953.