Yankee Metal Products (right here in Connecticut) made a lot of aftermarket parts and accessories, many of which were bought by the automobile dealers and recommended or even required by some manufacturers. For the most part, these were items to be installed by the dealers prior to delivery of the cars. Some early Mustang luggage racks, racks for British sports cars, mirrors for lots of different models, etc,. were all made there. The stock Triumph TR6 bullet-style mirror (also a 'holy grail' item today) was one of their products and was assigned a British Leyland manufacture part number. Much of their product line consisted of pot metal castings which pitted very quickly but that was common back then. Because of this they made a lot of everything so as to have replacements available.
I suggest looking in some British car forums and web-sites as most of their products ended up in that market. They also had factory catalogs depicting the different items and IIRC, some of them were available in peroid J.C. Whitney catalogs as well.
Nice find Mike. There are likely more out there, perhaps what I remember about the company and their product line / distribution will help others track them down.