If it is a "CLUB GUN", it would have the sn in all the normal places. I have 12 CLUB GUNS, and they all have serial numbers. I have two with just 0 and 00000 for sn's. The sn should be on the frame, barrel, extractor, cylinder and under the rear sight(target model only). The other possibilty is if it was confiscated by a police officer and had been used in a crime the sn may have been removed by the perp.
I recovered an old S&W Safety Hammerless from a stolen car that had the sn removed from the butt. It still had four other places where the sn was.

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Don Mundell