National Geographic : 1950 Jul
You Can't Miss America by Bus Bewildering Code on the Blackboard Means Work and Pay for Extra Drivers When a regular operator takes time off, a substitute is assigned to the run and his name moved to the "Stand" column. Relief men temporarily working regular runs become "Hold Downs"; hence the column "H. D ." These clerks direct operations in a San Francisco dispatch office. Usually a crowd of spare drivers waits and watches.