The problem is jurisdiction and venue. If they are making parts on request for "MONTANA" gun owners the BATF should be out of line.
If they are advertising nationwide and agreeing to send these parts across state lines they may be in trouble. The new law Montana passed was to protect Montana Gun Manufacturers from Federal licensing and was so they could make and sell weapons and parts to Montana customers only.
If the parts shipped will allow the assembly of a weapon that if made outside of Montana would require license then they cold give the BATF sufficient venue to support a raid with Montana State police assisting.