GB is correct, the seat profile is different, with the plunger seat pan dipping to follow the line of the frame backstays. The S/A seat pan is much flatter. Getting a new cover should not be too difficult, but there are variations, particularly the shape of the saddle nose. Try R K Leighton, Price Street, Birmingham...They've been doing seats probably longer than you have been riding, and can turn their hand to anything in the way of seats, repairs, foams and covers.
The mountings are also different, so for anyone looking for a seat, here's a quick guide.
Plunger has three bolt holes in a triangular formation at the front to take the support bracket, plus a pair of holes each side at the rear, brackets here mount on the mudguard supports. These pans are a bit weak, and benefit from some extra strengthening under the dip.
S/A front mount is two holes in line to take the support bracket, which fits into a pin located in the frame. Two holes at the rear for the support bracket on the mudguard.
eBay descriptions of seats are a bit of a minefield, as the S/A has several variants of the same basic layout. The plunger seat pan can also have holes in slightly different places at the rear to suit different model frames. What looks like an A10 plunger can actually be off a C11G or Bantam.
Swarfy.