The earlie engine back in the 30's had a drain back pipe in the rear cap, the later one like your 40 has the threaded hole but left out the pipe. Don't worry about it. Walt
If you install the return pipe make sure the threaded end does not screw in so far that it contacts the crank shaft! As mentioned it is not necessary but if you drive in an area with steep upward inclines it may be a good idea to have it so it can direct the oil returning to the oil pan away from the rear pan seal. This can help prevent an oil leak.