Tim is correct. The "D" in JAD designates the automatic transmission option in 51' manufactured at the Flint plant. This below resource says option 317. So, JAD decodes as a 51' 235 engine originally built for the powerglide option and available in any of the "Deluxe" passenger 2100 models listed here:
http://chevy.oldcarmanualproject.com/chevyresto/51042.htm

A pictorial of the models is here:
http://www.tocmp.com/brochures/Chev/1951/pages/51chevy2_jpg.htm


"Nothing astonishes men so much as common sense and plain dealing." - Ralph Waldo Emerson