That surprises me.
As far as I can remember, I've only ever seen swap in British writing and swop in American writing.
Although that might be wrong, because the Americanism for swap is usually 'trade'.
( Newington, Tue 5 Jul 2005, 14:12, archived)

Swop is old Brit
as far as I've ever seen it - 19thC and pre-war.
( Flapjack I spay a little mare for you, Tue 5 Jul 2005, 14:24, archived)

Hmmm
Okay, I'm probably remembering things wrong.
edit: swap is still the correct modern English spelling IMO.
( Newington, Tue 5 Jul 2005, 15:01, archived)