When the watchmaker Rolex introduced "oyster" into its advertising, what did the term refer to?

Question

When art competitions were introduced into the Olympic Games, the categories were literature, painting, music, sculpture and what other?

Answer 1

A new waterproof wristwatch case
Rolex records the oyster case as its invention of 1926, as "the world's first waterproof case for a wristwatch thanks to its patented system of screwing down the bezel, case back and winding crown against the middle case".