Restitution is generally not waived if there is actual restitution. Yet, the order you quote seems to say it is.
You should contact the lawyer who represented you in 1993 and send him or her the letter ASAP. If that lawyer is no longer available, contact a criminal defense practitioner who handles federal matters. The lawyer would contact the office that sent the letter and get to the bottom of it.
The lawyer will want to know when you got off of Supervised Release (or Probation), what the original restitution order was for and in what amount, your history or payment or non-payment and any other communications you may have had with the government on this. She or he will also want to have the original Judgment, Restitution order or Probation order. The case is too old to have the underlying documentation available on PACER so you shoal try to find the documents in your papers. If not, the lawyer can contact the office issuing the letter and try to get copies there or at the Clerk's Office. Obvioulsy, the more documentation you take to the lawyer the easier, and less expensive, it is for the lwyaer to get to the bottom of it.
We cannot give legal advice in this forum. I learned long ago that you should not dig your heels in until you have all the facts. You may be right but a lawyer's call to the agency will clear it up one way or the other.
Seems pretty clear from that statment that restitution was waived - in this case. I assume that you are no longer on probation as well which means the court lost jurisdiction over you for the purpose of ordering restitution as a condition of probation. You would, however, still be responsible for the special assessment in the amount of $350, plus any penalties or interest that may have accrued since then if you didn't pay it in full.
Call the agency that sent you the request for payment and try to explain it to them, or get clarification. Send them a copy of the order, and maybe that will do the trick.
The first sentence of number 2 seems to indicate that the door is open for restitution if the probation officer deems it appropriate. Was someone out some $ based on your actions? If so, you may owe something? Your attorney at the time should have the answers.