lboos,
Your gun still looks great! I never thought I would be able to work on a Model 10 trench gun project but with repro parts now available, it's the closest I'll ever get to one! I'm glad to hear you had no issues with your bayonet lug. I recently bought a barrel that the owner "believed" was for a Model 10 trench gun but didn't know for sure. I took the plunge and picked it up since he wasn't asking the moon for it. There are no grooves but there is a small recess area near the end of the barrel for what appears to be a set screw of some kind. After looking at the way the correct adapter fits on the Model 10, I saw that there was indeed a set screw on the bottom of the adapter. However, it still has the three screws running across the bottom of the barrel. I still have no idea f these are meant more for "clamping" and the set screw is what holds it in place, or if it mounts up the same way other adapters do.

As far as my barrel is concerned, there's no sight bead, is cylinder bore, is the correct length with no visible signs of being cut down an also falls within the correct serial number range. For all I know it may or may not be, but regardless, it will work for my purposes. I hoping someone who knows more about these will chime in and hopefully be able to tell me some more specific information on how the Model 10 bayonet adapter was affixed tot he gun.