Transfer Case Identification

I'm having trouble determining the type of transfer case I have. I've checked with a couple of web sites and according to them it could be either a D-20 or BW 1339. It's a '77 J10. The number on the transfer case ID tag is 300596-126 5357021.

Is it a steel cast case or aluminum? If Cast steel, its a Dana 20 if aluminum its a Quadra-Trac

That is in order to identification of case we need to know if it is a steel cast or an aluminum.