I don't know who came up with the idea of using the parts from the 507, but I thank them for thinking of it for me.
You are welcome.
While you are at it the 507 Base with the VTA adjuster also fits the 501 if your 501 doesn't already have it. It is quite course in operation but useful. I use it with a bronze stabiliser I custom built that goes under the armboard - friction fit plus 3 tapped screw holes to mate with the VTA Base.