Rulers related to typewriter and ribbon & carbon paper companies are a good sized category. Three of the ones listed here have calendars on them: How-Ko–1909/10 & Smith Premier–1910/11 and a Marchants Calculator 1959 calendar.  There are 21 rulers here and they are made out of 4 different materials: wooden, tin, celluloid and plastic. If the backside is not pictured, it means it is blank. They range in size from 5 1/4″ to 18″.

Click on name for picture and further info.