I wondered this too Chris. The only reason I think there were ever 3 is the protrusion of the stock bracket into the wooden stock at the position where the "4th" split plug could've gone.
The spacing is a bit off from side to side and doesn't quite line up due in part to what I believe is because of the stock bracket protrusion.
I posted a pic of the screen used one and a pic of my replica....the spacing is what gives me the idea that there never was a 4th plug. Although I must admit that I have a fourth one as a "backup" just in case new discoveries are made 
Steve
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