There were several Colt revolvers chambered in .22lr including the Cobra. However, you are probably talking about the Diamondback which came in .22 as well as .38. This is arguably the most popular or at least sought after of the Colt .22's.
By the reference to "mouse gun," I assumed the poster was probably seeking the smallest Colt revolver. The Diamondback is one of the larger of Colt rimfire revolvers, built on the same frame size as the Courier and Cobra followed in size by the I-frame rimfire revolvers, such as the Official Police, Trooper and various Officers Models. I suppose the largest rimfire is the Single Action Army.
I do not think the number of Diamondback rimfire revolvers equals the number of other rimfires, such as the Peacemakers. I also do not think it is the most sought after rimfire. I suspect the Bankers Special and Courier are far more rare and sought after.
More description of the revolver would be helpful.
These days the Diamondback seems awfuly sought after in 22. It certainly is not even close to be rare from a production perspective. The 22 Bankers Special and Courier are far more rare than the Diamondback in general. When most people think of a Diamondback, they think of a 4" Blue revolver. Rarity in the DB comes with the other variations.
There was a 22 Colt Derringer manufactured in the 1960's although it is certainly not a revolver. It would be better referred to as a belly gun.
About the smallest .22s that I can recall,in "recent years",that Colt made,were the derringers that .22 rimfire recalled from the '60's. They were single shot,side swing,in .22 short,I think. Sold mostly as "commemoratives",I think they were called "Lord & Lady".Not "toys" or replicas,but fully functioning guns.
Last "small" or "mousegun" .22 I can recall,are the 3" .22 Cobras,made up to the early 70's. I have one,ANIB,and while not a powerhouse,it will "do" in a tight spot,using "Stinger"hollow points and eye and nose placement.
To me, the term "mouse gun" means a small caliber, small gun, often carried instead of a larger, more powerful gun due to concealment issues or the unwillingness to lug the larger, heavier gun all the time.