the hammer seems to line up like a rim fire. doesn't look like a revolver. Almost looks like the item on the side would hold the bullet in place by spring action..like a reverse extractor.
.17HMR is smaller, but obviously not available when this gun was made, I imagine that's what you meant...
I agree, most likely .22 caliber, single shot, single action.
Below is a picture of a little one I have amassed over the years...some similarity...
The octagonal base near the chamber end is like a J. Stevens style .22 but the lever thing on the side is not right.