You're right, it's not a first model or second model or even a third model. It's a fourth model, probably in the 156xxx serial range. Not a three digit number 156.
Buck,
I'm not willing to call it a 4th model. Unfortunately the hammer is forward against the firing pin. That is the best way I have found to tell the differences between the hammer area profiles of the 2nd, and 3rd/4th models. I know the 4th is supposed to have an even shallower dip than the 3rd, but I just can't see it. With the firing pin back it gives you a reference point and your eye picks up the dip difference. I think the perceived difference between 3rd and fourth models is probably from different finishers and not the machining.
Now I can tell the 2nd from 3rd and 4th models, you can too if you look at the firing pin extension position.
I looked in my Madis book yesterday and he doesn't talk about the low serial number rifles. But, they are a reality and Parsons talks about them and it looks like he says 1868.
THE FIRST WINCHESTER John E. Parsons ; p102 " ...A few specimens of the Model '66 have been reported with serial digits of three or four from 100 to 2611. While no explanation for their separate numbering is known, the frame characteristics of such pieces are later than those in the 14,000 range, yet most have inside serials which would date them in 1868, before serial 20,000."
Being before 1868 would most likely make them 2nd model frames, the 3rd models seemed to appear around 1870. But, as always, they jumped around and in some cases they find lower numbers coming out of the factory years after higher numbers.
I'm not disagreeing with you except for the reported serial number (unless it is an outright fake), but what makes you believe it is a 4th model other than the sight positions and the barrel band I pointed out?
Regards,
Mako