I would guess from your very vague description that you have a set of Roadmaster baseplates. I would guess that they are model #165-2. As Dutch said, without pictures it's really hard to identify. You can Google Roadmaster and look it up in their online catalog or Google Roadmaster 165-2 and probably get several hits with pictures either way. I said 165-2 because I have a set that came off a Colorado but an earlier model. jd is spot on and is basically what I said.
There are several adapters and which one you buy depends on what tow bar you buy.
Good luck.