The brown seats were used on the '79 and '80 models only, and were flatter overall with a steeper, shorter cowling wedge at the rear.
You can graft a later seat from the '81 thru '84 models onto the earlier bikes, but they're not very commonplace to find used & you need to also replace the rear tool "box", possibly the hinges too.
You can also wait around for a Corbin, Sargent, etc. to pop up, and hope you get lucky on the price. That's what I did, but again, doesn't happen every day, or even every 6 months.
Personally, if it's just the brown color or a torn-up cover, I'd replace that with a black cover and be done with it.
Supposedly the earlier, flatter seats were more comfortable than the later dished ones anyway.
