The “50” in the part number refers to ANSI size 50 roller chain. The “R” refers to riveted chain construction. Riveted chains use a sidemash on the pins to hold the link plates in place instead of a cotter pin or spring clip. The extra digit refers to the number of strands. 50 and 50 1R are both single-strand chains while the 50 2R is a double-strand chain.