You do not have to mark the gun until it is actually assembled as a SBR. It can't be made up as a SBR until you get your approved Form 1 back. I had a Short Barreled Shotgun that I didn't mark for 2 months after I received my Form 1 back because I didn't cut the barrels until then. There is nothing that says you can't mark it before you get the Form 1 back. There is some difference of opinion as to when it is best to mark it. I normally I wait until after I get the approved Form back. I have one special circustance AR that I marked before I sent the form in though.