How do you know it was "de-registered"? Regardless, if you submit a letter to "take it off the registry", alll the ATF does is make a note of the permanent change.
I would guess, if a letter was truly submitted for that, you could do another F1 for a SBR, and not worry about any extra engraving. If someone ran a serial number check, it will come back to that gun and that manufacturer as a SBR.
Also considering the firearms brace fiasco with hundreds of thousands of F1s specifically with no extra engravings, I question most engraving at all now.
Considering you're chances of someone who knows what they're looking at (ATF) are virtually nil, if someone were digging that deep, you've probably got bigger problems than the gun.