The Excalibur is one of those planes where the fuselage is built upside down on the top block. It’s a nifty construction method that Joe Bridi employed extensively with his planes (although this is not a Bridi design), and it’s also seen in a lot of other pattern planes including the New Era.
The Excalibur looks to me like a miniature version of the New Era, which was published around the same time. It has a thicker airfoil, and a relatively longer tail moment. The plan shows an OS 10.
Let me know if you want the ribs notched for tail wheel or nose wheel.