1974. If the plan is dated "1974" it was probably in service long before that.
That's back in the diesel days. They went 75-80 MPH, were covered with silk or nylon, and could crash into 6" grass without even breaking a prop. Mechanics could do a one-flip restart and be back in the air in two seconds.
Guys would bring THREE models to a contest, just in case one might be damaged.