I stumbled across this quite by accident. About 11 years ago I worked as a soils engineer, working on a new building in San Francisco. Workers had to dig 3 ft below the deepest proposed footing, in accordance to the soils report recomendation.
They exposed an old sewer/storm drain, that was made of bricks. Some were tapered to accomadate the curved radius of the line. I saved many of those stamped bricks, knowing they were pre 1906 earthquake rubble. In those bricks were some stamped "Snowball", "Carnegie", "Pluto", and others.
Someone posted a "Carnegie" brick on a motorcycle web site and I did a search on the others, finding your post here. Isnt that a small world with just using your fingers. DSo I joined this site just to post this.
Additionally I found Chinese opium pipes, porcelin doll parts, glass artifacts, granite head sculptures and more.
As a side note, I viewed the map where Snowball bricks were located. Now I have history & contact with a relative of these bricks.. AMAZING !!