The U.S. Assay Office would produce what became the highest denomination U.S. gold coin to circulate, octagonal-shaped $50 gold coin-ingots, often referred to as "slugs." On the reverse, the design appears as a series of concentric circles in the center, surrounded by overlapping lines or perhaps a better way of putting it is a series of wavy, concentric circles (found on the reeded edge slugs). These concentric circular lines are known to mechanics as "engine turning," the design is similar to the web-like engraving in the vignettes on paper money or on watchcases, and unique to Territorial Gold struck at the Assay Office of Gold in the early years of the Gold Rush. The Kagin-6 Humbert "slug" was a small stepping stone along the West Coast economic evolution from simple gold dust to 1854-S double eagles. Its K-5 immediate predecessor also had a reeded edge, but its fineness was 880 thousands. K-6 increased the fineness to 887 thousands. The scrollwork reverse design, an anti-counterfeiting measure, was retained from previous varieties.
 
A compromise of sorts took place with a coinage act of Sept. 30, 1850, establishing not a branch mint, which the California business community had requested from the U.S. government, but a federal Assay Office of Gold in San Francisco, authorizing issue of ingots of $50 to $10,000 value, "to be struck of refined gold, of uniform fineness, and with appropriate legends and devices, similar to those on our smaller coins with their value conspicuously marked, and the inscriptions LIBERTY and UNITED STATES OF AMERICA." This wording concealed what the California authorities and Mint Director R. M. Patterson well knew: The ingots would circulate as money, with inscriptions conforming to the Mint Act of April 2, 1792, differing primarily in denomination from normal federal coins. As a stopgap, the $50 slugs served their purpose. Hundreds of thousands of them were produced, but subsequently a huge percentage of the issue was turned in to the federal Mints and melted.