On testing some of my cameras before selling, I realize that there are two different series of Industar 26M: - Regular ones, mounted on the early Fed 2 and 3,without click stops on the diaphragm mount, - The 1967 series (with the "67" prefix), mounted on the Fed 2 anniversary (with the 67 prefix too) which have click stops, like on the Industar 61.
So, no doubt these 26M were made specially, probably from spare parts...
The Industar 61M, with the 67 prefix, were mounted on the other official 50th anniversary series: Fed 3 and 4.