Thanks Hoyt! Amazing how easy something is to see, once someone points out that it's right in front of you...

As for the mystery hole... I'm guessing group number 0.686 in the master parts catalog ( http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0149.htm ) may be the answer: COVER, Clutch and Flywheel Housing Inspection Hole

1953 Chevy 3100
Father and Son project
In the Stovebolt Gallery
46 GMC 303 1.5T