The Trust is making and registering a non-NFA gun into a NFA SBR. The Trust can not make and register a NFA SBR out of a gun it does not own. The Trust must list it to own the gun. So I always list the regular gun on my Trust Schedule A before I send it in. That way the ATF knows the Trust owns it.
That's my line of thinking anyway. Who knows how the twisted mind of a bureaucrat thinks.