A flatbed was never a factory option in the 3100 series. Mine is either a 3608 or 3609 depending on how it left the factory; I'll never know.

You can see part of the front of a factory flatbed in this drawing: http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0095.htm

If you look on the site long enough you will find more drawings.