That one would've been an Autolite HRT 4001. It's black with a vertical fuse and the wires are held on with bolts. That was the one used on all 1947-50 KF products and that would mean it was also used on the '51 Frazer. In 1951, they went with a Delco-Remy unit which was a silver box with an es*** plug on it and horizontal fuse.
I recently sold an NOS one on ebay and I think I only got about $60 for it. The Autolite box said it also fits: 1952 - 1955 DeSoto, Dodge, Plymouth, 1946 - 1950 Willys, 1947 - 1951 Frazer, 1947 Nash, 1949 - 1950 Kaiser.
So you may want to search vendors who sell parts for those cars too.