While I don't have exact answers about what this piece might be, I did find info that might help fill in some gaps. Please forgive any typos; it's a bit late.
The Humboldt Cabinet Company was indeed located at 423 First Street in Eureka. They advertised regularly in the local paper at least between 1952 and 1977 (unfortunately the years before and after this have not been digitized). The relationship to "1928" is uncertain. According to a source I found, the building was constructed in the 1850s. 1928 may have been the date that the company started occupying the building, but that's speculative. However, in 1977 when the company was still located at 423 First Street, the owner of the building was none other than Norton E. Steenfott:
Norton Eldon Steenfott (referred to as "Nort" in some sources), seems to have been something between a local historian and a bit of a passionate oddball. I couldn't find much about his early life, but he seems to have lived nearly all of his life in Eureka, born either in 1912 or 1914 (the latter seems more likely) and died in 1985. He is described as having been employed in construction while also maintaining an interest in Eureka's past. In 1967, he was mentioned in the local paper as having an extensive collection of antique whiskey bottles in the window of Cooper's Pacific Pharmacy. In 1968, he appears to have bought the remains of the Eureka Foundry which he then sold off in pieces to anyone looking for steel, lumber, finishing material, or just curiosities. In 1971, a few years after his wife passed away, Steenfott was a founding member and program director of an organization calling itself "The Relic Accumulators,' dedicated to local history and interesting "relic" collections. He seems to have dedicated much of the rest of his life to preserving whatever he could of Eureka's history, buying and restoring buildings near Eureka's historic town square, and giving historic walking tours of Eureka's Old Town. The lack of digitization limits me to seeing just up to 1977, but I was able to find this picture of him:
None of this tells us anything about why this medallion exists or what its purpose was. Sadly, without documentation or a clear history, most exonumia is doomed to obscurity. Hopefully the context here might give you a direction to go in for more answers. Thanks for sharing it and good luck!
