A friend and I both enquired about this - he still has the whole machine, and is/was of a mind to keep the base for sentimental reasons but sell the horn! I sent him a reply politely pointing out the folly of this and urged him to keep them together, whether he ends up keeping the whole instrument or sells it. He thanked me and said he'd look into it...
OK, let me put it another way, the only one I've seen in the flesh had a very small 18 and a bit inch diameter bell. It was rough around the edges though. Could it have been cut down to hide the damage at the circumference? Who knows? But you're right, Frank's book does indeed say 22" so that must be right. How does the seller get it to nearly 24" though? maybe he's doing the reverse of what I did, namely measuring from 12 o'clock to 6 o'clock with a horn that has become distorted to an 'oval shape' at the bell and sagged a lot. I have seen horns that measure considerably less when you measure from 9 to 3 o'clock across the bell.
I see the auction has ended, perhaps the seller has decided to keep them together? I must be lucky, my Mk IX had a bit of a droopy horn when I got it, but with a lot of time, patience and the building of a "hoist", I've managed to get it almost completely back "up" to where it should be, it still has a small ways to go, but it's certainly come a long way. It's also in cosmetically much nicer shape than many others I've seen on the internet. Easy to see by the design how these basically collapsed over time. I'm glad I was able to save mine!