The Oscar Steinbach factory was on Union Blvd in North St Louis, probably in Walnut Park. I went there with my family in 1957. We had a tour of the shop and explanations of the different actions that were available. We reserved a spinet, and it was delivered several weeks later. I believe it cost $550.
I still own the piano, though it's out on permanent loan to a friend whose son is taking lessons.
It's in good condition, though I did have to have the plastic components of the action replaced in the 80s. The technician said that this was typical.
If you find out in the city records anything more about the company, please post it. I'll try to locate the documentation for the piano -- it's around somewhere -- and that should at least have the exact address.
I hope you get much pleasure from the instrument. I'm glad to know there are still some of them around.