OP here. Many thanks for the replies.

I just ordered a gunsmith screwdriver set so I can remove the cylinder assembly without buggering up the screw. This gun is just too darn pretty to mess up. Needless to say, I did not remove it when I did the initial clean.

A few more data points:

The cylinder appeared to open & close normally when I inspected the gun before buying. The same when I cleaned it with M-Pro 7, then a light coat of oil. The gun wasn't very dirty at all.

With a great deal of effort, I can push the "plunger" down with my thumb. When released, it sloooooowly creeps back out.

OldChief, thank you very much for your offer. Will PM you.

And so to work ...