This comes from the dec 10 release that superseeds all earlier releases, see my post on page one.
Gunsmithing
A dealer is “engaged in the business” of gunsmithing, as defined in 18 U.S.C. 921(a)(21)(D) and 27 CFR 478.11, when he/she receives firearms (frames, receivers, or otherwise) provided by a customer for the purpose of repairing, modifying, embellishing, refurbishing, or installing parts in or on those firearms. Once the work is completed, the gunsmith returns the firearms, and charges the customer for labor and parts.
you will notice in the above "fiearms" is defined as (frames,recievers, or otherwise). That definition aplies to any place you see the word "firearms". You will also notice "recieved from customer"
As with an individual customer, a licensed dealer-gunsmith may receive firearms (here we see firearms which alos includes frames and recievers by definition) (properly identified with a serial number and other information required by 27 CFR 478.92) and conduct gunsmithing services for a customer who is a licensed importer or manufacturer. A dealer-gunsmith is not “engaged in the business” of manufacturing firearms because the firearms being produced are not owned by the dealer-gunsmith, and he/she does not sell or distribute the firearms manufactured.
In this section you will notice the first 5 words, this section refers to lic. gunsmiths who do contract work for a lic. manf. "as with an individual customer" means it applies similarly to the first paragraph above reffering to individual customers. So according to this release a gunsmith can do contract work for a lic manf who supplies the parts. But the below section must also apply.
Once the work is completed, the dealer-gunsmith returns the firearms to the importer or manufacturer upon completion of the manufacturing processes, and does not sell or distribute them to any person outside the manufacturing process. Under these circumstances, the licensed dealer-gunsmith is not “engaged in the business” of manufacturing firearms requiring a manufacturer’s license.
This defines the requirement of the gunsmith vs manf. definition. The firearms once serviced by the lic. gunsmith MUST be returned to the lic manf. for "sale or distribution". A gunsmith is not a manf. if he does not build the rifles for sale or distribution.
So the clear cut description is the "sale and distribution" clause. If you modify any rifle or build from a reciever any rifle and then put it up for sale on you showroom, web-site or forum you have just made yourself a manf. thus requiring a manf. lic.
If on the other hand you assemble component parts into rifles, BY REQUEST of another, and that person OWNS the firearm(reciever or frame) then you fall under gunsmithing.
I believe the reason for this most recent ruling (dec 10) on the subject was to clear up the many past confusing bullitens on the subject and the mass of questions the atf was getting on a daily basis about this subject.