Had to stop and re-read this thread. Lol.
Transporation Form:
https://www.atf.gov/firearms/docs/f...rily-export-certain-nfa-firearms-atf/download Best solution, as the guys have stated above.
Form 1 is when you make something with what you own. Making an existing lower into a SBR lower.
Form 2 is the manufacturer reporting they made an item and registers it with the ATF.
Form 3 is dealer to dealer. FFL to FFL.
Form 4 is dealer/FFL to buyer.
You can engrave it at anytime, but have that done before or after the Form 1 is approved. Once it's engraved and approved by the ATF in any order, only then you can make the rifle complete. Might as well add laser engraving is the cleanest way. Make sure the engraving is .003 inches deep and has a font size of 1/16th of an inch at minimum. It can't be hidden and has to be done on a fixed nd non-removeable part. Can go on the outside of the magazine area where the manufacturers have their info. Lately I've seen it done on the magwell opening lip if it's a flat exposed area. Top of inside the trigger area. Bottom of the trigger guard if it's a integrated one.
I would also see about SS refunding the tax stamp or to further complicate things, see if they'll just forward that Tax Stamp money to the ATF on your behalf. Take a partial credit even or store credit. Sucks, but whatever they decide you're kind of at a loss on that one. Worst case you have to buy another suppressor. Just don't change your mind and sell it before taking ownership this time. If they refuse, maybe file it as fraud with your Credit Card company. They collected for a Tax Stamp that should go to the ATF. Pretty sure they shouldn't just hold onto money collected as a tax without actually surrendering it to the agency is was collected on behalf of.
Also make sure that the lower transferred to you from the FFL is transferred as a receiver or a pistol. I haven't seen any transferred recently as a rifle, but make sure anyway. Mistakes happen, those mistakes can be the cause of headaches. "Receiver" can be built as a rifle or pistol. "Pistol" can be converted into a "rifle". Use common sense there. Like don't record yourself doing it and throw on a buttstock before swapping the upper.
Proper order would be from pistol configuration, throw on the 16"+OAL upper on, then replace the brace. Or separate the upper and lower then swap parts around.
If it was a lower that was sold to you and transferred on a 4473 as a complete rifle or "rifle" lower, this lower cannot be made into a pistol but can become a "SBR". Taking possession of the lower as a "receiver" or "pistol" keeps your options open.
This allows you to built it as a pistol with the short barrel. Just rock a brace. So than once your form is approved and the lower is engraved, you can just swap the brace for a stock.
Different calibers and configurations...
To my understanding, you can use the SBR with different uppers. But you "should" keep the original approved configuration. I'd even always keep them together to be safe in a highly unlikely situation. If you decide to get rid of or change the barrel length or caliber to the designated upper for that SBR lower, that's when you are supposed to notify the ATF in writing. This is to notify them of permanent changes. Request acceptance confirmation in writing from the ATF in return. Swapping uppers is not permanent. If you don't have that upper you used with it, then have another of the same caliber and length. It's one of those ridiculous cover yourself bases for stupid rules. I don't have any experience with how petty the ATF is. So this is just what I believe to be the best correct practice.
If I'm wrong, someone correct me.
