I have a 14 page Trust that was created by a silencer company when I purchased silencers from them.  I'm now thinking of e-filing a Form 1 to create an SBR and would want that to be on the Trust as well.  Is adding an SBR to the Trust as simple as writing in the Make / Model / Serial number of the lower receiver on the "Assignment to the XXX Trust" to include in a .pdf Trust upload to the ATF?

Itemized schedule stays with you...

As long as you file the form 1 in the trusts name, and do everything needed for that process (everyones prints, photos, etc), then yeah. But you wouldn't add the SBR to the schedule until after it's approved. Because it doesn't currently exist until the ATF approves it. So it could cause issues if you submit the schedule that includes the SBR you want to make with the form 1 application thats obviously not been approved yet.

My trust doesn't have a schedule so to speak. There is no list of items owned by the trust that is part of the trust.

However, I have a page in my trust exactly as you describe where I "assign an item" to the trust. If I am filing an SBR I include that page with uploads when I do the eForm 1. I've done that twice so far and have been approved both times. The original copy of that page that I printed out and signed to scan in goes into the binder with the eForm 1 when it comes back approved.

My Trust works basically the same. There is no 'Schedule A' or list of assets that is part of the body of the Trust. All ATF or my FFL sees when I add something is the Trust itself, The initial contribution I made when I set up the Trust, and an Assignment of Property form. The Assignment simply takes whatever item I own, and transfers ownership to the Trust. Lawyer says to include this form with a Form1/4 submission to establish that the Trust actually owns the firearm in question. The Assignment gets notarized and dropped in the files. The ATF Form 1/4 gets printed/copied when approved and also dropped into the files. I have seven eForm 1s and a paper Form 4 so far. Nary a hiccup.

Not all trusts are written the same, so your mileage may vary. What do the instructions that came with it say about adding an item?