The guy on ebay is offering money back if you aren't happy, so you can order and see if it's what you want.
One big obvious difference in seats that I've noticed is that around '80 they stepped the seat more strongly. Look at pics and see which yours matches. I think that was done after '78, so he's probably selling the flatter cover.
My impression is a Special II is basically a Special minus some chrome - gauges, chain guard, maybe more. And I think wire wheels.