The two photos of the mail plane show a single exhaust, but I hadn't examined the photos in detail. When I did take a close look yesterday, I realized that the exhaust was on the opposite side from the usual exhausts, which led me to investigate further in the Windsock Datafile. The end result was that I discovered that the mail plane, and a few other single-exhaust examples shown in the book, actually had a different engine than the standard Bz.III.
So, out with the wrong engine! Luckily I was able to remove it cleanly without any collateral damage.

The most likely engine choice, from the table in the Windsock, is the Bz.IIIa. The other likely engine is the D.IIIa, as found in the WNW Albatros kits. I've got an Albatros, so I was able to measure the engine block and oil sump. The D.IIIa appears to be too long to fit, but the Bz.IIIa seems shorter. Fortunately WNW sells the Bz.IIIa separately, and I have one coming in from Weta. The engine ships from NZ, so the postage cost is about 1.5 times the engine's cost

, but the cheaper postage alternative would take 21+ days to arrive. I'd like to have this finished in time for the local GraniteCon IPMS show on October 16.
In the worst case, if the Bz.IIIa doesn't fit, I can drop the original engine back into place, and call it close enough.
On a positive note, I've done my first-ever rigging. It's just a test to see how it looks, but at least now I know I can actually thread those tiny tubes

. The upper tube is 0.5mm dia., and the lower one is nested 0.4 and 0.6mm tubing.

I've modified the rigging attachments based on this thread:
http://forum.ww1aircraftmodels.com/index.php?topic=5189.0Meanwhile, while waiting for the new engine to arrive, I'll work on the floats and wings.
Peter