I did some online research and couldn't find anything specifically on 'RG or GR' snap fasteners.
Dave
"The ‘snap fastener’ is a pair of interlocking discs, made of metal or plastic, commonly used in place of traditional buttons to fasten clothing and for similar purposes. Different types of snaps can be attached to fabric or leather by riveting with a punch and die set specific to the type of rivet snaps used (striking the punch with a hammer to splay the tail), sewing, or plying with special snap pliers. Snap fasteners are a noted detail in American Western wear and are also often chosen for children's clothing, as they are relatively easy for children to use.
Some of the oldest snap fasteners and snap-together connectors were made of metal and developed for the Chinese Terracotta Army dating back to 210 BC. Modern snap fasteners were first patented by German inventor Heriberto Bauer in 1885 as the "Federknopf-Verschluss", a novelty fastener for men's trousers. In America, Jack Weil (1901—2008) put snaps on his iconic Western shirts, which spread the fashion for them. The Prym Company has produced snap fasteners since 1903. Press studs were worn by rodeo cowboys from the 1930s on-wards, because these could be quickly removed if, in the event of a fall, the shirt became snagged in the saddle."