I found this article
http://books.google.com/books?id=8IZ...20G.29&f=false
It does say early G29's had no "G29" marking on the reciever, but did have the finger grooves, and the sling that can attach on both sides...
It specificaly said there is NO G29 markings when it is a 1939 with a 660 over it...and no clearance cut from the stock for the bolt knob...