It 's a Centennial Airweight;the frame is aluminum. The barrel says so on the right, so we know it isn't the all-steel one.
After model numbers were assigned in 1957, it was Model 42. The steel frame version was Model 40.
Collectors like them this old and will pay a premium over what the public will, for its utility value.
The hammer is internal, to avoid snags. You presumably know that you have to depress the grip safety to fire the gun. The current version lacks that grip safety.
The finish looks good in the photos, although the grips are a bit dull and need a coat of suitable finish. Some collectors recoil in horror at any refinishing, so I'll let one of them come along and give you a value range and comment on shining up the grips/stocks a little.
I personally will not buy one of these hammerless guns, as I like to cock the hammer for more precise shooting at longer ranges. I also think they're truly ugly. Others differ, and some will pay more for one than for a conventional revolver.
As a using gun, figure on $350-450, depending on how badly someone wants it. As for collector value, one of those guys will probably give you an estimate. Check back.
The factory does NOT advise using high velocity (Plus P) ammo in Airweight guns, or in all-steel ones made before model numbers were assigned. There are exceptions, but this gun isn't one of them.