Foxhound,
The file which you want has a 120 degree included angle. There are two types. One is called a cantsaw file and is flat on one side and has the 120 degree included angle on the other. The second type is called a websaw file and has two, 120 degree included angles, one on each side. This is the one that I have, and it's made by the Wood-Miller Saw Co., 5570 Ambler Drive, Mississauga, Ontario, Canada, L4W 2K9, Tel:905-624-2777. Their website says that they are suppliers to industry across Canada, Mexico, and the USA, so you may find a distributor near you. I just filed up the two blades on the NECG rear sight that I'm installing on my .450 #2 NE project gun and can tell you that it works beautifully! I find that the job is simplified if you make a shallow vertical cut first, with a screw-slot file. This gives you an exact centre guide. Widen that slit with a common 3-square, fine cut file, being carefull to keep it square. Then finish the job with the slitting saw. The toughest part of filing a shallow "V" is to keep the two arms of the "V" equal. I find that my system makes the job relatively easy, as long as you go slow and check frequently. You may find that Nicholson still makes the same thing in the US although it seems to me that their product range was dramatically reduced when they were bought out by Stanley.
