A 1916 advertisement for the Goldsmith Official League Ball:
The Peer of All
“Walter Johnson says: ‘It is the best Ball I have ever pitched.’
“The only officially adopted League Ball played under the NAtional Agreement.
“Guaranteed for eighteen innings.”
The 18-inning guarantee and mentions of the leagues which had adopted the ball for use were a staple of Goldsmith’s advertising, like the one below from 1912, announcing that the ball would be used in the United States, Pacific Coast, and Western Leagues:
The 1912 ad used the same image–that of Honus Wagner–that appeared in the company’s 1911 sporting goods ad, which quoted Wagner: “Your baseman’s mitt and Professional Glove at hand and they are my ideal style of a glove.”
By the 1920s, the 18-inning guarantee became generic throughout the sporting goods world
Not to be outdone, the Goldsmith ball of the 1930s was “Guaranteed for 36 Innings:”
Baseball History Daily 
A rubber-centered ball in 1912? Sounds like those leagues got gypped.