Interesting TTAG post about engraving

[asmo]

If you manufacture a NFA device from an existing firearm, according to the ATF you DO NOT need to immediately engrave your information into the firearm. Much like how firearms manufactured from a 80% lower receiver don’t require engraving upon completion, the ATF believes that the mere act of manufacture of an exiting NFA device doesn’t require additional engraving. The existing serial number is sufficient for identification.

Show me the ATF policy letter.