Model 12 and 42 ribs are soldered on. The original rib tooling and bar stock matting machine are in NY and you can have a rib installed for $695 including bluing. Most of the loose model 12 ribs come from a hot tank blue. The same issue that doubles encounter. Remington 31 guns are made from a solid pice of steel as you say.
Here is the shop with the original tooling.
http://m.ebay.com/itm/262782124326?_mwBanner=1
I have had 2 ribs installed by them on unusual 30" 16ga and 20ga guns. They did rib install only and my gunsmith did all the finish work. If you use them ensure the barrel is straight and concentric before and after. They can do the solid rib install on a 42 as well but the rib will not be correct to original Winchester, 42 solid ribs tapered to the muzzle and they can not do the tapper to the bar stock.
One more thing of note.....as I understand the matting machine is not a roll die, it is a alternating cutter, cuts the angle to the left and then to the right, gives the zig zag pattern.
One option for the cheap fix is acetone and epoxy on the loose rib, I would not recommend it but it has been done if the rib needs stabilized.
As you might guess I am an old model 12 junky!