OK, I got it sorted out. I went through the whole folder-delete/installation/patching process again, and fired the game up. Went to HL, tried to join AOP, and got the same message" using a different version". Oh, I'd loaded the History mod, along with the Carrier, and the texture mods. I then went to the ACES website and saw that only the History mod is being used! So, I tried once again, and with only the History mod enabled, and Viol'a! (Walla! in true American), I'm in AOP! At least I got to practice the process (several times).
All is well.
Flyby out