One thing I have found with older Barnack Leica cameras and the M3 is that sometimes you want to use both a light meter and a finder at the same time. These cameras and the M3 have finders that don't go wider than 50mm, so I needed an adapter onto a hotshoe that would mount both. Voigtlander makes and sells a double coldshoe and Bronica make a double hotshoe for their RF645, but both are quite expensive - over $160 - a bit much for what it is.
I had some tripod and flash mount parts in my basement, so I used them and constructed my own. My metal working skills aren't wonderful, but it does work. Here it is with the my Leica IIc, Jupiter 35mm lens, Canon 35mm finder and Konica meter. A brand assortment to say the least!