[DavidG]

There should be the number 5 between the letters AB and the three zeros. If so, the engine would #999 or lower (*AB-5000999* or lower), which is sort of interesting as Ford produced over 45,000 engine and transmission assemblies before the first one was installed in a chassis and there is evidence to suggest that Ford did not withdraw those Model B engine/transmission assemblies on a first-in, first/out basis. Your stamped number suggests that it is likely your engine/transmission assembly was drawn from that inventory on a FIFO basis.

I'm betting that your center cross member does not have the holes in it for the usual V-8 steady rod mounting locations close to the opening in the center where the rear transmission mount attaches like the one shown in the photo below. (It has the exagerated recess on the top of the cross member for battery clearance like the one in the photo.) If so, I believe that this type of frame was used only with the Model B engine and not with the V-8 which used a steady rod that attached on the front of the cross member rather than through it as shown in the second photo below.