Hello, I'm new to this site.
The "S" enclosed by an octagon is the mark for Standard Steel Wheels. The Standard Steel Castings Co. was from Cleveland, OH but its wheel foundry was in Clearing, IL. The latter is a suburb of Chicago. It took me a while to dig up any information on Standard because it's such a generic name, but I know marques like Hendrickson and Fageol used wheels made by this company up to about the mid-Twenties. I've also seen that "S" mark on Mack and White truck wheels. Since these two brands made almost all of their own parts, wheels on these trucks might have been made to the brands' own specifications. The names "Mack" and "White" are often cast into wheels. However, I don't think these truck builders ever made wheels themselves. Instead, Mack and White farmed out wheel production to companies like Dayton and Standard. Two other big wheelmakers were West Steel Castings out of Cleveland and Smith Wheels of Syracuse, NY. The only wheels I have seen that look just like your Standard are on Mack AC's from the late Teens/early Twenties. Some Whites used five-spoke front wheels from Standard or Dayton as well, but White-type wheels have less metal around the hub and more webbing where each spoke meets the felloe. Mack wheels beyond 1922 or so are more like this, too. Macks have a flanged, three-bolt hubcap, which is what your wheel would have had. Whites use a smaller, screwed-on hubcap.
--Neil
