Nice coin. I always have a tough time with these, but digging around in OCRE usually provides an answer, or a near-answer. According to OCRE, there are 18 (!) galley types for Constans from Siscia - so it could be one of these: http://numismatics.org/ocre/results...portrait_facet:"Constans"+AND+fulltext:galley Some of these have weird symbols in addition to the usual letters. Here is a start - note the various symbols for this one. Others on CT are way better at interpreting these symbols than I am: http://numismatics.org/ocre/id/ric.8.sis.199
The symbols are just the officina number repeated in the local language. If you look hard enough you will find a rare example where the Siscian number is larger and in the field as here with A or one.