@FrankFanboy
IF everything is as you have said in your post:
1. You severed your public entity employment which was NOT covered by the SS system in 1991
2. You received a refund of your contributions to the CSRS probably plus interest when you left this public service
3. You did NOT return to public service that was outside the SS system during the rest of your working years.
4. You receive NO pension from public service NOT covered by the SS system
If those things are true then then you have done correctly by requested a Reconsideration -
See if you have some sort of proof in your files somewhere - this form should be in your Federal employment file and I am sure they gave you a copy. SF 2802 (Application for Refund of Retirement Contributions)
Beginning in January 1984, some government employees began paying into the SS system - I am wondering if the confusion with SSA might be because of this. They call this the CSRS OFFSET. It is their confusion; not yours. Your refund of contributions should have ended your "vested" status.
FEDSMITH.com - 04/03/2019 - CSRS Offset: An Explanation of the Basics
But this might be what they are checking out. So this link is just to let you know why there might be some confusion with the SSA.
Good Luck dealing with them on this - hopefully they will get it right after a short investigation. IF Not, get help from your Congressman's Office.
Come back and let us know how it all comes out -
It's Always Something . . . . Roseanna Roseannadanna