since you obviously don't own a service manual.. i scanned the relevant pages of mine for you. describes it and has a pic showingthe straight edges... etc.
way wasier to look at than to tell someone over mail.
here are the 3 pages from the fo-4 service manual that cover setting timing on a front mount dizzy ( 9n/2n or early 8n )
you really should invest in getting one of these manuals if you are gonna use that tractor.