1928 Auburn 6-76

New to this forum.  I am the proud owner of a 1928 6-76 Auburn, fully restored in 1986 and inherited from my father, George Rawson.  My father passed away in 2006.  I am trying to determine where the oil dipstick is on the straight 6 Lycoming Engine.   There is a brass plug below the oil filler tube at the level of the crankcase but no way to remove this plug.  Can anyone help?

Thank You,

Kerry Rawson