4 speed identification

The gearbox retainer, assuming it's the one that the main shaft exits through on the front, does fit within the hole in the BH. It doesn't mate up flat because of what I assume is the secondary shaft that protrudes underneath the main shaft. I don't see a hole for it and don't know if is supposed to be inside (see new pic above). Wouldn't the casting numbers (listed above) tell us if it's supposed to be in a car?

I downloaded a manual for the '66 Dart/Coronet this morning. I'll spend a little time with it to see what I can glean relevant to the questions above. I suppose I also need to download the '75 version.

I notice the tip of the U-shaped fork looks broken (see last pic). Is it supposed to look like the tip of the "?" fork?

The seller stated it was from a passenger car and not a truck. People say whatever. He also stated it was a 3-speed OD box. Applying your instructions, it looks like a 4-speed box.

There is another trans available from the same guy. Aluminum case. No other information on it at this time. If it's in better shape (not so much broken stuff), I'll try to work a swap using guilt and hassle as bargaining chips.

Thanks for your help on this, Sensei.