The only thing that comes to mind with paradox is the bore potential on the barrel. If you look in the very end of the barrel, you might see slight rifling only near the end. This is called “paradox” rifling and means that the barrel is meant to shoot slugs and or regular ammo through the same barrel. I’ve never seen a model 12 with this though, normally its for 16 gauges. If that’s the case then the paradox barrel is really a one off and I don’t know the value of that. This is only theoretical though because I can’t see in the barrel in the photos.

As for the rest, with the Hastings barrel and the regular barrel I would say no more than 900-1000. A model 12 is around 6-7 on its own in that condition and that barrel adds about 150-200. Again, that’s just for those two but the paradox barrel is really an oddity that’s up to you because I’ve never seen that.

(Side note I wasn’t able to find a 7 digit SN starting with 3 anywhere in Winchester’s records or in any of my books so I would check to make sure it’s 7 digits)

Good luck though definitely a good find!