In Pietta parts bin,there only seems to be one grip frame,and most that I have seen I could
be wrong have the shoulder stock mounting notch at the bottom of grip frame at least on brass.
But thats Pietta 
Just to reiterate (ad nauseum for you frequent folks), Pietta changed the 1851 Navy Type grip profile beginning with date code [CN] 2015 to include all models (1851 Navy and G&G). Prior to that, ending in [CM] 2014, the grip profile was of the "tail" variety.
The Pietta 2014 gripstrap, trigger guard, and wood parts are not interchangeable "individually" with the Pietta 2015 (or newer) gripstrap, trigger guard, and wood parts. As a 3-piece unit they can be fitted easily to any Pietta 1851 Navy .36 built after ~2000.
Yes, almost all of the latest Piettas have that bottom gripstrap divot on both the brassers and the steel models. I managed to acquire a gripstrap without the divot (through no fault of my own) from VTI (you might contact VTI or Taylor's for inquiries about Pietta parts) who sold it as an 1860 backstrap when I was venturing into putting an 1860 gripframe on an 1851. I have given up that quest for now.
However, the divotless backstrap now resides on my G&G.
As an aside, I have always admired the 1851 squareback TG. I ordered one from Taylor's 3 years ago, for the 2014 1851 Navy .36. It took about 3 months to have the order clear customs, et al. It fit perfectly on the 2014 "tail" pistol. I don't think Pietta is offering any 1851 Navy Squareback pistols these days.
If you want one you might want to check Dixie Gun Works. It will most likely be a pre-2015 model, but those will start to be collectible in repro circles since Pietta is not producing them anymore.
