That's familiar and would make sense with how I remember them changing from a 530 chain to a 520 o ring one. I'd forgotten that until you mentioned the sticker.
Current Bikes CB250RSA x 2 (one from new in 1981) VT500FT Ascot 1984 Canadian CL175K6 1972 USA CB250N 1978 CD200TB 1985 (reg) CB450K6 1973 USA XS650S 1981 USA