Hey, everyone, just wanted to provide an update and share my experience with everyone else; hopefully I can save someone some trouble somewhere down the line.
After I went through my supply of regular caps, I ordered a pack of 29mm and the larger capping bell from AHS. They came in today. The new bell goes right onto my Red Baron, the thingies slide out and reverse, no problem (they're even marked 26 and 29 on the other side! very handy) and I take one of my many 16.9 Zywiec bottles, put a cap on top, power the Red Baron down onto it, it crimps the cap, and.......
The cap falls right off the Zywiec bottle.
Apparently 29mm is too large for the 500 mL bottles I've been using - they must be some really weird in-between size. I'll probably just keep using the standard caps; they don't fit quite right but 98% of the bottles are sealed well - once in a while I open one and there's no hiss and the beer is bad. But I like the bottle size too much to give them up.