Lots of those overdrives were installed in British cars with the electrics set up so the OD could be used on both 3rd and 4th (or 2nd and 3rd on some of the '50s cars which were 3+OD), and there were even a few on which you could select OD in 2nd, 3rd, and 4th. What is vitally important about that switch is that you must never, never, never engage reverse while the OD is on -- that will do disasterous harm to the OD.

I remember in the '60s listening to MGB racers argue about whether to take certain turns at Donneybrooke (now Brainerd International Raceway) in 3rd OD or 4th direct, etc. They couldn't even agree on which shifting was better for the durability of the trans.