You'd have to take a close look in person and review what catalogs can be found on the issue. The catalogs themselves don't always show or list everything that existed, but it's better than no guide at all. If the bracket is an old addition, done by a skilled framebuilder, and the bike has been painted since, it can be very hard to tell if factory or not based on photos. I would lean towards aftermarket addition, but "never say never" with these old lightweights because you could custom order them if you really wanted something added. The easy ones are the new changes or sloppy ones. The ones that are hard to tell are the ones done a long time ago by someone who knew what they were doing. In person examination often reveals more than the online photos. It's an interesting project but I would not throw big money after that.