If the engine is original, look on the spark plug side of the engine. There is model, type, and code numbers stamped into the blower housing. The first two digits of the code number tell what year the engine was manufactured.
Yup, 28086S I know it stands for 28" 8HP Series 6 with electric start. I need to decode the serial # to see if that tells me the year, manufacture plant, etc. Trouble finding a decoder for that...
You deffinatly have a 6 series, that was the first series with the foot operated parking brake. I'm pretty sure the 6 series started in 1987. What is your model number? 2808 something?
Also the engine, if the original, i think it has the year in the model number.