I'm not far from you. It is hard to tell what the barrel contour is. I may try to get to a local dealer and look for a comp.
Is the brake threaded on or integral? Any idea of round count? I know you said used.
I believe the break is removable i can get the caliper on the barrel and get a measurement for you. Christansen has the barrel contour listed as feather weight if im not mistaken.