[zeeba neighba]

This vintage would also have the much reviled internal lock.

I wonder if the Colt King Cobra also has this. I've got one that won't open.

[Tijeras_Slim]

Nope. Extractor rod may have unscrewed, a problem pre 1961 Smiths had occasionally. I'm not entirely sure about Colts.

[Rockpile]

I had a K frame model 38 keep unscrewing it's extractor rod thus being real hard to open the cylinder. You can dis-assemble and apply a SMALL AMOUNT of Loctite to the threads.

Colt calls the same part an ejector rod. Don't recall offhand which way it's threaded, could be backwards.

Another possibility is a burr on one of the crane lock detent parts in the recoil shield {where the cylinder locks to the frame at the back}.

[B4Ranch]

Check and see if you can turn the extractor rod. Possibly it is loose and needs to be screwed back in. I recall having the same thing happen many years ago.

I wrapped it with a cloth and tightened it with pliers. Problem gone forever.