I think a Dutch cap is a diaphragm. I don't know why it's thought to be Dutch.

Perhaps you were thinking of the French Tickler?

Those European nations are pretty much interchangeable anyway, particulary since the formation of the European Union.

It's sort of like saying 'The (French,English,Spanish) Disease' when referring to syphillis. Every country takes credit for the good things and tries to associate bad things with their rivals.

Although in this case it might have been due to the shape resembling an article of headwear associated with the Dutch.