Have no ideal, I just know prior to that time retirement was after twelve years, today it is after one full term. And the change was made while republicans ran the congress under newt.
The problem is the number of years you have to have to achieve "vesting". The minimum time for vesting in both systems is still 5 years. That would mean that you would get 1% of your high 3 for each year of service. So, that would be 5 years of service times 1% times your average high 3. You also have to be at least 55 to Qualify for that ~ (with exceptions for RIFs or disability, when the percentage disallowed might be lowered).
So, let's say you have a Congress critter who meets the minimum age requirement, and he has a high 3 of $170,000, and he has 5 years service time ~ so he gets 170,000*5*.01 = $8,500 per year FULL RETIREMENT PAY.
Luxurious eh?
If he can live until he's 82, and he's 56 this year, he gets that for the next 26 years. Not counting COLA, and who gets that anymore, he could get $221,000 and then die.
The big changes from CSRS to FERS occurred in 1985/6/7 under Tip O’Neil and the Democrats.