Hi Scott, and welcome to the sport!
I will start with the age of my 16. It is supposed to be from the early 70's and it has a stamped number on the posts going into the hulls. Stamp number is 1163. That is the only number on the cat and it is what I used to register in my name. I would appreciate any input.
The Hobie 16 was introduced in late 1969, and the first method of serial number placement was just as you describe.
This continued until late 1972, when the practice of engraving 12 character hull identification numbers on the transom was mandated by federal statute.
It's unknown what the first production serial number was, nor what the "final" one was before Hobie Cat shifted to the newer, required system, but 1163 is indeed a boat from the early '70s. It's a testament to the durability of these boats that they're are still in service 40 years after they were made.
(please feel free to let me know the proper name of these posts)
They're called "pylons".