Eric,

I was just looking at my Dane sniper a bit closer.  The barrel on mine is the same diameter as a quarter, maybe very slightly less.  So, yours sounds about the same.  The front sight looks the same.  The rear sight looks generally the same, with just slight variations, like the graduations are a bit different.  Mine starts at 0 and has markings up to 15 (going from top to bottom on the sight) with lines between and then a large gap with 250 toward the bottom of the slide.  My marking line on the left side of the stationary part has a V arrow in the back of it.  The knobs are located the same with just slightly different looking markings.  It looks as though there is a little bit of a notch out of the magazine to make room for the front adjusting knob on yours.  Rather interesting.
My guess, from looking at yours and the markings on the barrel, is that it is the original barrel on a civilian target rifle that someone just bored out and bored the chamber to make it a 45-70.  Probably in the US, since the ammo is much more common than the 8x58R.
Still an interesting gun.  Set us know how the shooting goes.

Best regards,
Tom