Ed,
The only other thing I have found out is that the NP date code uses no letter prefix. So it looks like we can safely say that the #11-21-60 numbers that you have listed is Nov 21, 1960 which would be correct for a '61 model truck. The other number(s) could very well be casting numbers and my 2-Ton Chevrolet Parts Book or any other book I have does not list them. We need someone to drop by that has Transmission knowledge or Transmission books to help.
Bill
