Even though Rod Dinkins is an SK now, his son Jeff maintains Rod's web site, and that has a pretty good
history page, at least up until the date Rod passed away. From that page:
"Effective July 1, 1976, any Extra class licensee who had been a licensed Amateur for 25 years or more could select one specific 1x2 call sign. This added the ability to pick a specific call, but did not change eligibility.
Effective October 1, 1976, anyone who had held an Amateur Extra class license prior to November 22, 1967, could select one specific 1x2 call sign.
Effective January 1, 1977, anyone who had held an Amateur Extra class license prior to July 2, 1974, could select one specific 1x2 call sign.
Effective April 1, 1977, anyone who held an Amateur Extra class license prior to July 1, 1976, could select one specific 1x2 call sign.
Effective July 1, 1977, any Amateur Extra class licensee could select one specific 1x2 call sign."
Note that each of these "seniority" dates has significance.