I'd say your 636 is an early production unit with a Rev. A mobo, these only had one RAM slot, and no video in/out slot (the slot that the LC630 DOS has, which it uses for video input from the DOS card, and is used on the 5x00 series Performas for the Apple External Video Connector, i'm not talking about the Video System card slot, all 630's have those)...my LC630 is a September 1994 unit, one of the first, and it has the Rev. A board. I think they're fairly common.