New and old. The entire grip frame has to be used. You can't use a new back strap with an old front strap. I have seen them on ebay.
Hawg is entirely correct.
I have a Pietta 1851 Navy Second Model SB TG (date code [CM] 2014 "tail", similar to Hawg's) and a Pietta Griswold & Gunnison (date code [CN] 2015, similar to Hawg's). I have placed them butt-to-butt as a slightly different perspective to his photos and the dimensions are very different insofar as the 3-piece grip execution (backstrap/TG/
wood) is concerned, as Hawg pointed out. Bottom of post.
I also have a Pietta 1851 Navy Third/Fourth Model (date code [CP] 2016, not pictured) which has the same grip profile as the G&G, unlike the Second Model.
If you procure them on Ebay (or similar sites), just be sure the 3 pieces match. Don't think that the "tail" wood can be just sanded down because the newer style backstrap will protrude by very much, thus the thicker wrist. It will save much heartache when you have them in hand. Been there, done that, and have a few parts in the spares bin as part the learning process.
Here is a pic of a newer backstrap on an older TG. No joy! (Pardon the faux chrome screw: just for display positioning purposes!). I guess some enterprising individual could sand down the front strap of that TG, but would also need to fit wood to it. And, if that is the only TG one has on hand, it will only conform after the sanding to a newer backstrap, and one might just save it for posterity or bartering material and get a TG that fits the backstrap. Lots of work.
My perspective, FWIW:
I find the highly disdained "tail" grip profile with the thinner wrist to be a more pleasant feel to my hand, whereas the newer rendition has a much thicker wrist.
I have seen YouTube videos (like Mike Belliveau's) about modifying the original Pietta "tail" and though it is much work, as the OP stated, the finished product has a thinner wrist than the newer Pietta grip profile, which I find preferable to the newer profile. IMO, there is no way to thin the wrist on the newer style.
If you are looking for a singularly "correct" original Colt 1851 Navy grip shape, there are none, and I would refer you to Nathan L. Swayze's 1967 treatise " '51 Colt Navies" for many references/photos to see that this is so.
How do you know whether or not you are getting "old stock" when you purchase?
If you are speaking of complete Pietta 1851 Navy pistols, look for the date code. Any pistol date coded [CN] 2015, [CP] 2016 or later should be good to go. Insofar as parts on auction sites, buyer beware as most folks on EBay part out pistols bought from Cabela's and have not much clue as to what original repro date it came from.
Take care and watch your money/purchases.
To each their own. Good luck to you with the newer profile!
Jim