AE 16mm.

[Phil A]

Cant pin this one down.
16mm 1.67g
S in left field SMAN in ex, I think.

Thanks

[Victor]

Your coin is Constantius II with a SPES REIPVBLICE reverse. With the dot S dot in the left field on the reverse, the mint must be Sirmium, so your mintmark should be either ASIRM or BSIRM.

[Phil A]

Thanks for the help Victor. This one baffled me because it's the first time I saw this reverse type. I take it they are not too common.
  Phil A

[Victor]

They are not common, but not that rare either...maybe scarce.