Tim - I'm not sure what you mean about not liking the AACA answers. My only goal is to learn as much about this car as I can. I don't like your insinuation to the contrary. I posted the photo there last night and here this morning. I posted here too because this is a very active forum and is focused on Plymouth and Dodge.
Regarding AACA, I thought it said D3P at first because that is what it looked like the first day I saw it before I put some steel wool to it. The P13 you mention was given to me by the owner and he mistakenly took that number off the head. Keiser asked for a photo and it took me three weeks to get it. Now I have it and the code is pretty darn clear. What's it mean? I don't really know and have no agenda other than to figure it out.
To Andy and Ed - Until Andy suggested otherwise, I did think all blocks had codes. And I do know what the proper code is for a 1942, but that's not the point. I want to know, if possible, where this engine came from. If it came from a 1934 Special, for instance, it might have some value to someone with that car.
Neil - Thanks for your input and for not taking my follow-up question as an insult. It certainly was not intended to be one.