[Daves55Sedan]

I looked at a '54 Merc station wagon in a junkyard some years ago and it had the medium case Ford-o-Matic in it.
Easy tell-tale sign is that the Merc medium case had a bump-out on passenger side of pan that the small case trans does not have, and, the medium case fluid pan has a drain plug at the front driver side of the pan. The medium case is a little longer than the small case, not that you can tell with the naked eye unless you have both sitting side by side.