Looks like sept 28, 1856 but could be 1858.Welcome to Tnet.
You have a patent date on the buckle, so no need to resort to buckle-dating charts.
Only certain combinations of day/month/year are possible for US patents since they take their date from publication in the Patent Bulletin which has traditionally been published on Tuesdays. Your picture isn't good enough to enlarge very much but, taking into account it has to be a Tuesday, it looks like the date might be Sept. 21 1858. Can you confirm this or say what the date is?
If it is September 21 1858 then congratulations! You have a buckle from one of Cornelius Campbell's patent trusses (there are no other buckle-related patents for that date):
View attachment 2049525
If I have mis-read the date then it's something else, but can still be traced even though we don't have the actual patent number.
Looks like sept 28, 1856 but could be 1858.