Now, when you say it was the first production safety seat, do you mean it was Yamaha's first? Because my dad's '81 Maico 490 actually has a safety seat. It's the only one I've ever seen on that year of Maico, but apparently it was an option at some point. If it's an aftermarket part, it's the only one on the entire bike.
Your dads Maico one would be an aftermarket job, either ceet of Hy Flite (I think), Maico got a "Safety Seat" in '84
1982 YZ's were the first bikes to have them standard from the factory.