The original IBM PC keyboard just used an arrow. The keyboard that shipped with the PC AT used Enter, while the enhanced keyboard (Model M) used an arrow for ISO layout keyboards, Enter for ANSI keyboards, and I think some variants for non-PC platforms used Field Exit, which was also used by IBM for many keyboards before the PC. I'm not sure what other vendors may have used, I'm sure there are some return keys out there, and for all I know IBM did ship keyboards with Return instead of Enter.