breezy, Below are some snapshots form Chevs of the 40s website showing their descriptions of those two different rear engine mounts. From reading those descriptions, it appears the left pictured mount in your post should be for your 3/4 ton (3600) truck. The right hand one seems to be identified as for the 1/2 ton (3100) trucks.
I am away from my parts books so can't get into them for more info. Others should be along to chime in here to confirm or refute my info.
