Often, two rubber rings are used on the rounded top two inch flippers.
This had to do with the advent of the flipper during the time when the standard pinball was 1/16" larger in diameter. The flipper was not changed when the standard ball size was reduced, and to compensate, double rings were recommended on the flippers. When the flat top flipper was introduced, it was designed for the smaller ball, and so only a single rubber was needed.

Absolutely not. The game was designed for the smaller ball. It's only the flipper die which was never modified for the smaller ball. It doesn't make much of a difference, but I do double the rubbers on the round top 2" flippers.