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They must have had some way to do it. I’ve seen a few Martin catalogs that specifically say that other sized bells are available on custom order. If it was a huge pain, I doubt they would have mentioned it in the catalog, but rather grumbled a bit then did it for ‘special’ customers only.

King cut 19” upright bells down to 16” for William Bell and Abe Torchinsky for their rotary CC tubas.

However, you’re right, my “trivial” comment was a bit off.

I'm not nitpicking. I really don't know much, but I've been told that by someone who watch tuba bells being spun and talked to those bell spinners about different diameters of pancakes.
The farthest I've ever been onto the European continent was the Netherlands, and Adams was only making drums back then.