http://www.6066gmctrucks.org/spicer-5831-b.htm

http://www.thedieselstop.com/archiv...ollapsed&sb=5&o=&fpart=1.htm

Your looking at a Brown-Lipe transmission. I don't have anything on an "M-43". My guess is that stands for Manual trans 4 speed main, 3 speed aux. Brown-Lipe was a supplier to GMC through '42 at least. That's where my info ends. I do have a number of models listed. If you find a 3 digit or 4 digit number (numbers-no letters) post it and I might have some info for you.