While there is some rubber inside a pneumatic golf ball, its not a true "rubber core" ball, because its center core is air!

This “The Pneumatic” golf ball is all there, but most of the original lettering on the poles is lost.  Enough of the "Goodyear" remains on one pole to positively identify that name.  Fortunately the word "Pneumatic" still remains oni opposite sides of the equator so the ball can be positively identified.

The Goodyear Pneumatic ball was covered under a patent was filed on Sept 11, 1900 by Addison T. Saunders who assigned half of it to Frank Seiberling, the man who founded Goodyear Tire &  Rubber in 1898. The making of this ball was a bit of a marvel in and of itself.  According to the attached 1905 advertisement, the wall of wound thread between the cover and the pure Para Rubber “jacket” that held the compressed air consisted of a single thread of Sea Island cotton over one thousand feel long that was wound with perfect accuracy and uniformity.  The cover of the ball was also made from pure para rubber.

In the center of the ball, inside the para rubber “jacket” is compressed air.  The accompanying ads show the core of compressed air and the other layers.  The first advertisement is a Goodyear ad from 1903.  The second was run in 1905 in the UK by the London firm of Geipel & Lange. Notice how Geipel uses Goodyear’s exact illustration of the ball.

This ball shows significant wear, but it's still a worthy ball. It has been used but with only one strike mark, and the lettering on the equators is faint but still readable.  Pneumatics are one of the most "creative" balls produced and not many remain.