The original 1920's Osborn consisted of eight large classrooms (one room for each class) on two floors. The bottom floor of the school was rest room space, furnace & janitorial room, two small meeting rooms, and some form of kitchen. The 1949 addition added eleven more classrooms; two for each grade, one for kindergarten (which was voluntary and had only about half the enrollment on the other grades), and two for vocational classes.
In the new addition there were two classrooms on the first floor #106 and #109, four on the second floor #214, #215, #216, and #217, and five on the third floor #324, #325, #326, #327, and #329. The eight original large classrooms were incorporated into the numbering system; #210, #211, #212, #213, #320, #321, #322, and #323. The interesting thing is that the first digit was irrelevant as the other two digits were unique to each room and ran sequentially from bottom to top, but having a digit representing the floor was probably useful.