That's strange, I order parts from them all the time and have not had them turn me down except for certain parts that they want to install themselves (Cocking pieces and parts that need to be installed buy there own gunsmith.
The reason they probably want the serial # is probably to make sure it is a Mark 5 action machined for one of the 378 based cases. If you want to use the 378 extractor on another action/bolt they wont sell you one because they want to fit the extractor themselves.
Just tell them you have a mark 5 in 378 and your smith needs to replace the broken extractor
The 30/378, 338/378, 416 Weatherby and the 460 Weatherby all use the same extractor if this helps.
J E CUSTOM