The maker's primary/#1 function is quality assurance/control. Most makers used out-workers on high/best quality guns. So, if the maker inspects the work done and says it is up to standard and you are welcome to put our name on it I see no problem. Who is to know if work is done by in-workers or out-workers. The bigger question is does the work meet the standard for the gun in question.
The catch in the above is if the buyer is not competent to judge the quality of the work and is not going to employ an expert of his own. Then he must rely on brand name work. "By others" has a slight taint to it.
DDA
