Well, I don't know what to think now. It is definatly a D and only a D. You can tell the block is perfectly level and the numbers weren't milled off. You can read the TI223D clear as day (deep stampings), and there is nothing after it and nothing to make me think there was ever anything there. Most paint appears original.

Yes, there is a road draft tube on the back. Is that good? I mean, what does that tell me?

The motor was put in in the early 60s, and I know it's never been out of the car sense then, so any machine work, or if the motor was ever taken apart, it would have had to have been done then. With the motor being relatively new at the time, it was probably just dropped in.

What else can I look for? I guess the next step is to pull a valve cover and see what I can see.