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Pure curiosity here... has Gronitz ever produced a rotary CC?

The answer is yes. I think there are a couple (at least two) here in Norway.
A former colleague of mine in the army had a 4/4 that he bought in the early 80's.
We went to the Gronitz' shop in Hamburg late in the building process to try leadpipes.
Dieter Kleine-Horst had three leadpipes ready but wasn't quite happy with how the tuba played and told us to come back the next day. He then had made a leadpipe he was satisfied with and that was the one my colleague also chose.
The Gronitz team is capable of making whatever you order. But they need an order.

Grontiz makes rotary tubas, and you could probably special-order a rotary CC, although there is not one listed at their website, only the PCM (Piston - C - Medium), or 4/4 size piston CC, and the PCK (Piston - C - Kaiser), a large piston CC. their website has photos and descriptions of the FZ125 and FZ126 (F - Zylindermaschine, or rotary valves) as well as their piston F, and their piston and rotary BBb tubas. I think the BZK looks like it would be a ton of fun to play. At about Euro 13,000.00, it would cost about $18,600.00USD, which is not bad for that kind of craftsmanship.

Very cool -- I'm sure it's an excellent tuba. I just happened to be looking at their Web site and it made me curious. I already have my handmade German rotary CC needs fulfilled.

The Gronitz team is capable of making whatever you order. But they need an order.

I was in the shop a couple years ago discussing F tuba modifications, and also discussing my own PCK while I was there. They will customize it the way you want it.