Does your boat have a metal tag just behind the splash deflector? If so, the serial number stamped in the tag can be compared to a chronological serial number chart.
For boats 1973 model year and newer the tag was dropped for the now required Hull ID Number. This number can be found pressed into the upper right-hand corner of the boat's transom.
Should you find your boat doesn't carry either a tag or a stamp it most likely had a tag originally and has since lost it. In that case, comparing your boat's features with other boats of known years will be your best solution. If you would like assistance making a comparison please put up some pictures of the boat.
The rudder's mounting hardware is one key feature since this was revised in 1972 to the one we see in use today. Hull colors and stripes help some as do details of the deck equipment such as the halyard block and cleat.
I too am trying to date my sunfish. There are two rivits behind the the splash guard but no plate. No number in the transom. What year did they add the compartment in the cockpit? Mine does not have one.