Situation: if some one were to have a "pistol configuration" with KAK or similar brace BUT said person legally SBR'd their "pistol" with a stamp.

Question: person could surely shoulder said "pistol"   BUT could they travel with it loaded? in a state where traveling with a loaded pistol is fine but loaded rifle is prohibited for hunting reasons?

Once a pistol always a pistol. But does the brace permanently become redesigned as a stock once it has been shouldered?
Can it go back and forth based on how it is used?
If I remove the stock from my buffer tube, my SBR, becomes a pistol again, since it started as a pistol, they brace thing seems questionable to me.

It would depend on State law. In Alabama for instance, any firearm with a barrel length of < 12.5 inches is considered a handgun, so I can legally carry my 5" barreled SBR as a pistol in the car.

We have a similar law here in Michigan. Any firearm, rifle or handgun, under 26" overall length is a pistol under Michigan law and must be registered (unless you hold an out of state carry license) and may be carried loaded with a Concealed Pistol License (CPL). Barrel length or presence of a shoulder stock are irrelevant. Also, Michigan measures firearms in the shortest configuration they can be fired in, ie. stock collapsed, and removable muzzle devices off.  Federal NFA SBR rules still apply.

Sorry, double post.

Until the stamp come back it is a pistol with or without the brace. Check your state laws on what your states definition of a "pistol". You might be surprised what you find out. In Alabama a pistol is "any firearm with a barrel less than 12 inches". Our pistol definition here in Alabama opens up a wide variety of firearms to be carried on person or in your vehicle with a "pistol carry permit".