It should be pretty easy to measure each wheel across the face with a pair of calipers - even homemade ones, and look for differences.
Measuring was actually the first thing I did. The wheels are within about 3/16" of each other in terms of overall diameter. That's not conclusive either way. The outer rim doesn't really determine wheel diameter, it's the inner rim where the tire is seated that makes a difference. Unfortunately, there's no way to accurately get a measurement without removing the tire. I was hoping that someone who has seen more 16.5 inch rims than I have would recognize it right away. I've attached photos of a wheel I found on the internet that looks very much like my wheel and it's confirmed as a 16.5. It just looks different--the shape of the outer rim is much flatter and there's a less pronounced corner where the bead seats.
