According to the Blue Book, they did make them in .22 Mag, but it is identified as "limited production". Trooper MkIII was also made in .22 Mag and is probably a little more common. /forums/images/graemlins/cool.gif
I have had people tell me they have seen one, but I have not. The parts books of the era DO have .22WMR cylinders and barrels listed, which give some support to the story. Although the .22LR Python was cataloged, it was never produced and the parts books DO NOT have any .22LR cylinders or barrels listed. You decide if the .22WMR Diamondback exists. I want to believe it does, but, until I see one, I will be skeptical.
Years ago I spoke to a person by telephone who was listing one in Gun List for sale. He said they exist in limited numbers, but I have never seen one. At the time, the gun was about 2x what a regular Diamondback was selling for and that seemed too high. Boy was I wrong.
They were cataloged. 22 DB production didn't start for a couple years after it was cataloged. My guess is that there are a few around.