Thanks for the responses thus far; they've been helpful.
However, I now understand my description of the problem was not clear. Please allow me to try again. I didn't think to take a photo and the machine isn't handy at the moment.
Looking at the distributor, all seems normal.
Unclip the cover and dust cover and look at the rotor and attempt a start and the rotor does not rotate.
Now, look more closely 'beneath' the bottom bowl-shaped housing of the distributor.
The distributor driveshaft has about 2-inches of exposed shaft visible. It looks like the hold-down clamp has come loose from the block and ridden up the distributor driveshaft, still clamped tightly to the shaft.
My guess---per original post I'm always slow to just start yanking parts off before I have an understanding of what's happening---is that the distributor driveshaft has 'risen up' away from the block, having elevated away from the engine internal mating gear.
No external engine damage is evident---just the 2-inches of exposed driveshaft, the clamp still tight but elevated, and a rotor not turning.
I hope this better explains what I'm seeing.
The replies thus far are sincerely appreciated. All insights and advice welcomed.
I'll continue the dialogue and add information as I dig deeper into this failure.