You show a Smith & Wesson Safety Hammerless revolver, sometimes called the Lemonsqueezer because of the grip safety.
It appears to be a 4th model, made sometime 1898-1907 but might be a 5th model made after that period up til WW II.
The serial number is on the bottom of the butt and will show which.
If the serial number has been removed (you say "no other markings"), the gun is illegal to possess, but I doubt the BATF helicopters will arrive to confiscate a hundred year old revolver.
The right grip is actually off a Harrington and Richardson, not original to that S&W.
Caliber is .38 S&W which is not the same as .38 Special or .38 Colt.
The auto is a P38, as marked, which is no relation to a Luger except 9mm caliber and use by Germans. AC42 means made by Walther in 1942. The Germans were in fair shape at that time and the gun is likely of decent quality. 5574K is the serial number. The little stamps are proof and inspectors' marks.