I dropped a 10mm end wrench in the space in front of the radiator of my 1975 BMW 2002 and when I got the rad out I had 2 10mm end wrenches and a socket.
25/64, 6 pt sockets are a borderline alternative for a 10 mm. They fit pretty snug, you may have to pound them on and pull them off the nut with a monkey wrench, but after a dozen uses, they hog out enough that that isn't necessary most of the time. Deep wells don't get lost as easily.