Hmmmm.... I know some things have changed recently due to ATF ruling 2010-1 with regards to temporary assignment of title 1 firearms, but I don't see how that would apply in any way here.
You bought a SW receiver which shipped to you as a serialized "pistol" via an FFL. You then sent your pistol out to someone to perform work on it and they are simply sending it back. I have done this many times and the pistol was sent directly back to me.