This isn't technically Thompson related, so feel free to move this to another forum as needed. I just hope that other people might find it helpful.
I recently moved from one city to another within the same county of the same state, and I took my 1927A1 SBR (and .45 suppressor) with me. Prior to moving, I found conflicting information online regarding what kind of permission/notification is required by/to the ATF for an *intrastate* address change, ranging from doing nothing at all, to submitting form 5320.20 prior to moving, to submitting this form after the items have already been moved. I opted to go the safest route and submit the form prior to moving the items; however, I did not know the preferred/recommended amount of time in advance to do so.
On Sunday, March 17th, I filled out form 5320.20 stating that the "dates in transit" will be March 24th-25th. I then printed, signed, scanned, and emailed the form to the ATF. Today, March 25th, I received a secure email back from the ATF with the signed/approved form, dated March 24th. So it would seem that as little as one week is an acceptable amount of time in advance to submit the form and still get it approved, though I suspect they would prefer more time than that.
Part of the problem is that form 5320.20 is entitled "Application to Transport Interstate or to Temporarily Export Certain NFA Firearms", yet they want you to use this form for permanent *intrastate* address changes too. Heaven forbid they create a new form just for that purpose and reduce the confusion.