Here's a reply I received concerning the serial number, which I received from another forum where I asked this same question.
Yes your "DR " number is a asigned ID number. Many years ago many states used both engine and chassis ID numbers on titles. So you got a GM new replacement or reman engine you had to go down to the DMV and get it inspected and the title corrected. Dad replaced the engine in our 41 Chevy in 51 and down to DMV we went. Most car makers stopped using engine IDs towards the late 50s and states started using just the chassis ID on title.
I'm pretty well convinced that the the 50 truck has a 52/53 216 engine. When getting parts I'll probably treat it as a 52 or 53 engine. I will go look for the casting date code.
