To my consternation, I managed to wipe out my bookmarks just now. Something about inadvertently starting the create new game process within the existing game, I think. So I went to look for user.dat and found that it had already been overwritten.
I am now building new set of bookmarks and the next thing to do with user.dat will be to BACK IT UP.
FWIW, on my laptop the file is located at:
C:\Users\bruce\AppData\Roaming\Out of the Park Developments\OOTP Baseball 25\config
Hmmm. That makes for three locations of vital files if you play OOTP through Steam: steamapps, documents, and appdata. Keeping track of everything is an experience with this game.