In the beginning, there was only DARKNESS!! Then ...... POP!!! There was Blair.
Sorry Blair, T4t and I sort of went sideways after we figured out an accurate report flat-side would be cost prohibitive to build.
Some years back, there were ALL of the Transitional rifles on the Rare Winchesters pages. There were in total, 7 known versions of the Henry while it was in transition to the final iteration of the '66. At that time, the rifle with the sliding brass fore-end was lumped in as
a possible 8th Henry Transitional.
since that time, the number of rifles displayed has really gone down. I don't know why, except perhaps, the owners wanted exclusivity.
Anyway, based on the most prominent design features, we could conclude the early and initial '66 Flat-Side, wasn't a '66 at all, but a natural progression of the Henry. I submit, the gun was built on a Henry Receiver. Flat loading Ladle added ala, the foregoing transitional, and the only major change from the original Henry, the barrel assembly. It (the '66) didn't actually "become" the '66 until the second model when the double dovetail side plates went do do, the scoop ladle was introduced and the flange for the fore-end was added to the base casting. Rendering the "Flat-Side" truly a "Henry Improved." Although I don't consider the two part Barrel/Tube Mag an improvement. Just a rust trap.
Coffinmaker