I think most of the OEM pumps were Rule 500 GPH pumps. They used a few differnt styles though. Some had the manual "Round" pump... some got the RuleMate oval pumps. (they were manual/automatic) And... I've seen a few of the old square style.
Post a pic of your pump, and we can let you know.
FYI... My islandia has an oval Rulemate 1100 in the rear... and a square Rule 1000 in the pond.