42RH in 68 Dart advice/request

I cannot understand how it is possible for that to work. I'm not saying that it doesn't, I just can't wrap my head around this. Overdrive is actuated by an electric solenoid inside the trans. I have no idea how you could manually shift into overdrive as there is no linkage attached to the OD system. Is it possible that this is what trashed things when you tried to use it? I think you said that you have never gotten the car up to speed to use OD, is this correct?

Do you have a lock-up torque converter and how is it triggered?