Sounds early for such a late serial
I believe ZDA is serial #803 the YY-#### serial was used for military Beavers (Models U-6A and U-6B) with the year making up the first part of the number.
I'm not entirely sure about the history of the serial number designations and how they were spread about the various models but I'm sure there are some people on these forums more knowledgeable than myself that may be able shed some light on this.