That's not entirely accurate. Colt started serializing the slides in the 30s some time. They stamped the serial number under the firing pin retainer plate for some time...into the 40s at least.
ETA: Found what I was looking for:
http://www.coolgunsite.com/pistols/...M1911s have only one,as well as the receiver.
Yeah, the auction I posted was a matching slide & frame. Still wouldn’t give $4k for it though.