A size of 10mm just about rules out
Justinian I.
DOC lists two examples the
Sear type SBCV 0197, and they have diameters 14mm and 16mm respectively. Your coin looks much more like those tiny nummi pieces which circulated in the mid to late 5th century before
Anastasius reformed the currency and got rid of them. They are often notoriously hard to identify precisely, but novacystis’ suggestion of
Marcian from
Constantinople is a
good one. Looking again the letter inside the
monogram does indeed seem to be an S as he indicates.
Bill R.