Little accessory piece turns out to be for Starrett protractor level from 1902 catalog.
Info on practical Machinist per grandson.
Will try to copy address he gave me.
http://www practical machinist.com/vb/antique-machinery.and history/elusive-starrett-No12-protractor-level accessory-3630601
Sounds like its a hard piece to find. Understand that as it would be easy to lose.