Could not find one in France. Wanted one. I ordered one and had it done by my gunsmith from my model 21 (.44 spl, Classics series) and the 4" barrel and cylinder from a model 27 pre-1982). The front sight was milled in the ramp (the original was too high). No more internal lock, and it is P&R ...
If I could go back to 1980 and buy any S&W, I would buy a S&W 520. No doubt. It would be cheating, cause I would be a time traveller and I'd know it would be a unique revolver. But yes, a model 520.
Forget the one beginning with the F, especially if it is not the same. The only one that is important is the one stamped at the bottom of the frame (N XXX....) ;)
The distance between the flat bottom and the top left of the left target K grip is 4.196". if it's longer, it an N...
The width of the same target grip (bottom, outside) is 2.407". The width of the top of the grip is 2.205" (outside)
On my pre-24 (N) : the square butt at its very bottom is 1.957" long and 0.357" thick.
On my model 10 (K), the square butt at its very bottom is 1.854" long and 0.334" thick, if it's what you needed to know...