There never was a lock box.
From the very beginning all money not distributed as benefits was spent. What was then done was keeping double books, pretending there were assets when there were none. Simply, the same entity cannot both own a bond and owe the debt it represents.
Just try to tell a bank loan officer that your right hand had loaned your left hand money and then say you’d an asset on account of the “bond” in your right hand. They will laugh at you while they are calling the authorities to turn you over for security fraud (assuming they take you that seriously).
… well, that or you’ll find the dumbest loan officer in the nation.
Again, there was no lockbox.
As for opening what never was, well, it’s more corrupt than that.
Under LBJ, not Carter, they stopped the double books. Had that been all they had done it would have been a great reform, for it would honestly admit that all was unsecured empty promises where future tax payers were on the hook, being defrauded.
But it seems that LBJ also started spending the black ink on the ledgers, ignoring that it was offset by matching red (again, the same entity cannot own and owe a particular debt), issuing more red ink.
They weren’t spending the same money twice, just pulling money out of thin air by borrowing it into existence, inflating the money supply in the process. As always, inflating the money supply robs savers by devaluing money. They used this notably to help pay for the Great Society and Vietnam.
The “black ink” ran out early in Reagan’s tenure and the practice stopped.
I will leave it to you to decide it it’s entirely a coincidence that inflation fell around that time, once the money supply wasn’t being inflated on the sly.