The 60's

Other countries lose faith in the dollar's value and exchange their dollars for gold, causing our gold reserves to get smaller.

Soon after John F. Kennedy's assassination, the Kennedy half dollar is minted.  Because of a silver shortage, silver becoms more valuable.  The Treasury starts redeeming silver certificates for silver bullion, not silver dollars, and makes fewer silver coins.

Silver certificates are no longer exchangeable for silver bullion, but they are still legal tender.

The Philadelphia Mint moves to its fourth location.