I went through the literature what separates a Gunsmith or dealer FFL (01) from a manufacturer FFL. According to that, the typical stock maker or gunsmith would not be considered a manufacturer. The only case that seems to fall under this would be if someone is building or assembling custom guns on a regualr basis as part of their business.
The example that ATF gave, was if a gunsmith is regularly buying surplus military guns and then sporterizing and reselling them as part of his normal operations, then he would be a manufacturer.
The occational custom buld or conversion does not qualify you as a manufacturer. It has to be a routine operation.
Or if you are operating as a subcontracter that is consistently doing one operation on a part to go into another part, then you would be considered a manufacturer.

B.Dudley