The synthetic cornice blocks were installed in the early 1970s to prepare the Old Mint for its new role as a museum. The original cornice blocks had been roughly removed during the 1950s, I believe, rather than maintain them as they aged and presented a hazard.
The Old Mint Museum opened to traffic in the fall of 1973 to coincide with the Cable Car Centennial celebration, and the official grand opening came the following year. It was closed in December of 1993 following a determination that the building was not up to the post-Loma Prieta earthquake standards, re-opened briefly for a few months during 1994, and then closed for good late that year. The closings always seemed to come around the holidays, when no one would be around to protest.