WOW! What a great find!
According to Ted Hake, from his book, "Official Price Guide to Disneyana" (available at the book shop at Disney-MGM Studios), your pin back button is from 1938. The estimated value is $50-100- 150, with the lower value being in "ratty" condition, and the $150 being near mint. Yours still has the original cardboard backing and that is very inportant!
During the 30's, there were quite a few pin backs similar to yours that were produced for advertising and promotions. They were all done in celluloid and used the same colors.
This is a wonderful piece of your "Disney" family history and I'm sure, it is worth more to you, than whatever it would get on eBay! How old was your grandfather when this was given to him?
VERY COOL!!!