Yup... Variable pitch indeed. An old Marx unit, 50 mm prop, 15 degrees reverse to 45 degrees forward blade pitch angle. These things have been discontinued for at least 30 years if not longer, but 7 years ago out of a clear blue sky I managed to find 2 new ones (both NIB, never used).
Pretty decent quality, triple bearing double sealed sterntube, the hub has 5 ballbearings out of stainless (one for each blade, and two in the pitch control mechanism). Entirely maintenance free, yet the hub is taken off in 5 minutes if needed.

I know you know a guy, but the issue for this carb is adjustability. Right now I can trim that air intake tube, and as long as it works, I'm not going to change that...
It took me 7 long years to come up with this idea (remember I mentioned once that a lot of my solutions are "brainwave-based" and that I have little control over where or when they pop up? This was one of those... )

THis is a more clear vid of governor response:


As you can see, I had to extend the throttle lever by a "huge" amount to reduce barrel movement, and mind you, this is a carb intended for a 1 cc engine feeding a 5 cc engine.
Throttle bore is something like 2,5 mm in total, and at 0 load, the throttle opening is even barely visible with the eye, at full load it is maybe 1/3rd of its stroke opened.
It's a weird set of parameters to work with, I can tell you...