I see that my reply above was clear as mud!
To clarify, FERS was effective 1/1/84, it just took til 1987 to get TSP up and running.
But even though TSP was not up and running until 1987, if hired after 1/1/84, Federal employees are FERS.
You can read about FERS at the TSP.gov website.
FERS retirement has 3 components:
1. social security, which is withheld from pay check
2. government retirement plan, which is withheld from pay check - not voluntary
3. TSP, which is like a 401K, and any witholdings from pay check are voluntary
So, while I can't speak for the post office (which may be different) or even the military (which may be different), when it comes to just a regular federal employee hired after 1/1/84:
The amount withheld from paycheck for retirement, for employees starting in 1984 or later, goes into a retirement fund managed by OPM (I believe).....and that is true for the withholdings before 1987 and after 1987.
GA