December 8, 1962, the college coaches create the USGF as a sport governing body for gymnastics. AAU is still the official governing body. Much more information is available at the USAG History Timeline.
The FIG publishes a new scoring system in 1962 to be used in the 1964 Olympics. The new system formalizes difficulty as part of the gymnastics score. The difficulty system is ordinal, with A, B, and C value skills, and the 10.0 score that was once solely execution is split into content and execution components. In the US, the old way of doing things (AAU and later USGF) was to adopt the FIG rules, and write extensions and interpretations for the USA, including the non-FIG events. While that was easy for relative scoring, the FIG had now specified a list of difficulty values, and someone was going to have to write that list for trampoline. Someone did, and it existed by 1966 (USGF Rules for Men, 1966-1968). I am still looking for earlier versions of those interpretations, and trying to determine who wrote them. Unfortunately, most of those who might have been involved with writing those rules have either died or are beyond remembering.