Thanks for doing this NWL. Rob - why bookmark? Just copy/paste and save for a reference. 
Ok, here's my addition. The pictured set is a #26 and came boxed but not with all of the items shown in the 1923 catalog. Based on characteristics of the set itself - the lack of a raised area under the cab where "A.F. 16" is cast, the mix of the truck types on the cars, and the lack of an observation I've always thought it was a 1923. The set may be but, given what you have shown it does not look like the tunnel is original to the set. There are no markings and the dimensions match the description of the #82 but the overall finish and mold quality don't match what you have shown for the early '20's.
The second item is my #261 Hudson Tunnel
Based on your photograph it looks like they must have had multiple molds for the tunnel (which would make sense given one-tunnel-at-a-time production would have been ridiculously slow) which varied slightly. I just took this picture and tried to match the camera angle to what you have. The valley between the peaks at the top of my tunnel looks to be deeper than yours, the house location and size appear to be slightly different and the base of the tunnel on the house side on my tunnel has more flare and actually makes the flare side longer than the opposite side. I set a straightedge across the peaks and had a go at measuring the valley depth - I get 5/8".
Inside
The only downside to your post is that it looks like I not only have to try to find a correct tunnel for the #26 set but that I have to start looking for another tunnel for my Clipper set. Since the Hudson tunnel matched the description of the dimensions for the tunnel in the Clipper set I figured the fact that #261 wasn't listed in the 1930 catalog was just another one of those Flyer things but I guess not. Oh well, so it goes. Again - I really appreciate all the time and effort you put into providing this information.
