I have found it on a lot of school buses and even public places that were built circa 1960s.

Charter Oak  Suggérée par donshottype 

URW's Charter Oak is a digital version
Charter Oak was originally produced by the Keystone Type Foundry, which operated in Philadelphia from 1888-1917.
Charter Oak is shown on in their 1906 specimen book https://archive.org/details/abridgedspecimen00keysrich at pages 143 to 145.
As you note the font was popular for bus signage in the 1960s.