I remember back about 1955 when Kadee couplers came out. About the same time the NMRA came out with their stupid design. I couldn't believe what the NMRA did. The Kadees were so slick and looked protypical. Oh, well.
The "horn hook" coupler was never an NMRA standard. There was an NMRA committee, which was completely outside the Standard Committees, that looked at a number of experimental (X) coupler designs, one of which was the X2F. When it was put to a vote, around 1955, the NMRA membership rejected the X2F (and several others) as an NMRA 'Standard' coupler. What was accepted was a standard coupler pocket, so a standard shank could be developed for the many couplers being used at the time could be interchanged between manufacturers cars.
The problem with Kadee's at the time was they wanted a licensing fee from manufacturers that would have priced most cars out of people's reach. Instead the manufacturers jumped on the X2F as their de facto standard coupler. Not because it was an NMRA standard, but because it was easy and cheap to make. The main differences being the size of the mounting hole, large for body mounts, small for truck mounts.
