The SSN-based seniority assignment began prior to the 2007 hiring wave (therefore, it had to be sometime prior to 9/11.)

An interesting anecdote: I was in the July 2007 class and it was brought to our attention at the ALPA pizza and beer welcome party (day 2 or 3) that the company had been assigning seniority incorrectly. I distinctly remember the announcement, "One third of you are going to be really happy, one third of you are going to be extremely upset, and on third of you aren't going to care." Suddenly the plug in our class became the most senior (went from 88B to 737B.)
The company then went back to early 2007 and reassigned everyone's seniority number within their class. Since there were mutliple AE's and everyone was on first-year pay anywhere, it wasn't a big hurdle.

Officially: The higher last four, the happier pilot.