Unusual banner tow

Saw an odd airplane towing a banner over downtown Denver today. It was
a biplane, with a higher aspect ratio than I've ever seen on such a
machine. And the leading edges were swept over about the outer third
of the span...it looked for all the world like two Discus wings
grafted onto a Jungmeister fuselage. Anybody know what this thing was?

Incidentally, the banner carried the logo of FirstBank, and read "This
is the closest thing we have to a private jet". And just to drive the
message home a bit more, the airplane emitted a puff of black smoke
every so often...

rj