I have built 4 sbrs. After a few years I had them all in the various configurations that I intended them to stay forever. In my mind they were permanent changes. I wrote a letter and let Martinsburg know the current and permanent destriptions.
About two years later, I got a personal phone call from a supervisor in Martinsburg. I had submitted a change letter and one of the changes had her very confused. I had changed both the barrel and the stock on one of the guns. That change changed the barrel length without changing the overall length. She asked that I write them again and explain exactly what I had done to change the barrel length without changing the oal.
She again asked me to explain why I was writing the letter changing the description of 4 guns. She said that was not necessary as there was no such thing as a permanent change to an AR15 platform. I said, "OK, but I don't want any problems when I am dead and you get Form 5s that don't match the Form 1s. She said good point, but again there is no such thing as a permanent change to an AR15 platform.
I wrote the letter she asked me too and about 18 months later got a letter confirming the change to the 4 guns.
So, no more letters for me. It just confuses them. There is no such thing as a permanent change to an AR15 platform, according to Martinsburg.
The only thing good that came out of the whole mess, is that I had a Form 4 pending when she called me. I mentioned that I thought she might be calling about a problem with my Form 4. She said NO, it had nothing to do with that. The Form 4 got approved about a week later. I have no clue if she had anything to do with that.
In summary, there is no such thing as a permanent change to an AR15 platform.