The original manufacturer was Ryobi. Any model number that starts with 315. Neither Sears or Ereplacement show that part available. It's possible that some other routers used used the rack that is on the motor and there are only a few common diameter motors so it's possible that a base from a different router might fit it. For example my DeWalt 610 has a rack and pinion adjuster and it might fit. That would give you the opportunity to find a dead router with a good base instead which could be easier to find.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.