Very nice piece of gold.

The Federal government wanted to open a branch mint in San Francisco beginning in 1853 but was beset with issues of finding a suitable property. In the interim the former Moffett and Co. minting facilities were purchased and beginning in 1854 coinage began to be minted bearing the S mintmark. Because of the more reliable purity and weight of the coinage from the San Francisco mint, private coinages began to decline markedly after 1855.

In contrast to later mintmarks which were microscopic by comparison, this mintmark on this coin is comparatively huge.