This is either an inspectors mark, or the English factory S stamp meaning unserviceable arms sold out of the service. Since England officially didn't support either side during the civil war, this was used by arms traders as a way to export them from England to the states without "officially" supporting the war efforts.
It is amazing after 150+ years in the ground, this rusty relic still has a story to tell, through the magic of electrolysis.
Building the display
fit. The third adjustment fits perfectly.