sourdough
40 Cal
I believe the shoulder stock aspect of the 1938 Federal Firearms Act only applies to cartridge guns.
Hawkeye specifically stated assembling it with a conversion (cartridge) cylinder, which would make in an SBR (short barreled rifle) in accordance with the National Firearms Act of 1934 unless the revolver has a 16" barrel or longer (Gun Control Act of 1968).
When replica BP C&B revolvers started appearing in the late 1950's, the NRA went to bat for owners of such revolvers with shoulder stocks, persuading the Treasury Department not to consider them as SBR's, even though the NRA refused to run advertisements in the American Rifleman for companies selling BP replicas of any type for a very long time. Don't take my word for it: William B. Edwards, "Civil War Guns", 1962.
Hawkeye also accurately observed that the Cimarron stock photo of the revolver was a cut-for-shoulder-stock with the "4th-screw" escutcheons removed. Easy enough to do, and if one does not like the looks of 2 empty threaded holes in the frame, 2 short set screws can be installed in place of the escutcheons. Just make sure they
As an aside, I don't like either the skeleton stock (aesthetics) nor the Pietta "remedy" stock (again, aesthetics) for a 3-screw gun. That is why my stocked guns are 4-screw guns using the Colt Type 3 design. I have an ASM 1860 Army fluted cylinder 4-screw in a cased set with a shoulder stock. The stock fits the Army grip frame assembly well. I also have a Pietta 1851 Navy 4-screw with the 1860 Army grip frame (not historical but they fit with the 1860 stock). Regrettably, I only have one Type 3 stock at this time, but I am looking for another at a decent price, rather than the going price of $275-$300+ per copy.
Just my $.02 worth.
Jim