Found some info that might interest!
The most common back mark found on hard rubber buttons (and other objects) is from the Novelty Rubber Co. which usually shows as "N.R. Co." abbreviated on buttons. N.R. Co. was founded 1853, incorporated 1855 and produced until 1886. Other marks found on hard rubber buttons and objects are: India Rubber Comb Co. (IRC Co.) and American Hard Rubber Co. (AHR Co.) and there may be others including "India Rubber". The "India Rubber" mark does not have the Goodyear patent information and may have been made in England or France or could be considered a "pirated" item lacking the required patent info. I know of one button style with this back mark, a cameo profile, one can assume this is an antique button due to the styling, it is not thought to have been made by any of the large companies noted above.
Hard rubber buttons will differ from other look-alike materials because they are back marked. Some dyed, pressed horn buttons are also back marked but they should not be confused. The company names shown above will appear on hard rubber buttons and objects, horn has different marks.
Collectors call hard rubber buttons "Goodyear buttons", Charles Goodyear held the patent but did not make any buttons. A relative of Charles Goodyear did produce pewter metal buttons much earlier than rubber buttons made an appearance, his name was Amasa Goodyear. Charles Goodyear's vulcanization patent expired in 1865. However, the Goodyear hard rubber patent of May 6, 1851 (covering buttons, combs, etc.) was extended (by Nelson Goodyear) in 1865 and did not expire until May 6, 1872. After the patent expired in 1872, the largest manufacturers still put their names on items they made. Many later Novelty Rubber Co. buttons are marked with only "Novelty Rubber Co." Additionally, the India Rubber Comb Co. (IRC Co.) used their name along with "Goodyear 1851" as a trademark and their successor, The American Hard Rubber Co., filed this as an official trademark in 1906 and used it at least through 1917.
