BTW, the shutter DOES NOT screw on to the shaft as shown in some illustrations, like this one.

(http://www.arkansas-ope.com/TECUMSEH_CARB_REPAIR.html

It appears to be attached by a tac-weld.

Here, I'm speaking of the old one, from which I wish to use the shaft and lever only.

Here is a link that might be useful: Carb Repair

This process could possibly be expedited you were able to post a picture of the carb lying on your work bench.
But, in lieu of that, I'll have to refer you to the image below.
Judging from your description of a "tack weld" securing the shutter (butterfly) to the choke shaft makes me wonder if that "weld" might be a dab of epoxy someone added previously to keep the shutter from "creeping out of place" during operation of the running engine.

Everything I can read from the manual implies that the shutter is intended to be "held in place" by 4 tabs, or "high spots" on the shutter. To remove the shutter from the shaft would require that you open the choke so the shutter is on edge, then using a suitable hook type tool, reach to the bottom edge of the shutter and engage the hook tool on the bottom edge of shutter. Then you would pull the shutter toward you forcing the 2 bottom tabs through the long slit in the choke shaft.
After the shutter was loose from the choke shaft, you pull the choke shaft out of the carb body from the side.

You can try to "scratch" the surface of the "weld" with a screwdriver or file tip to see if it is an actual metal weld or solder, or if it is epoxy.
It may be that what you see as a weld is actually the tabs of metal referred to in the text.

{{gwi:342229}}

Youda man, Mownie. What I thought was a tac-weld was the tabs on the shutter. I pulled it out, then the shaft. That's the old one. Now I have to see if the new one works the same way.

Yep! It doesn't close as tightly as I think it should, but it works, and that's the measure, I guess.

Thanks a bunch!!

Jim