This Siscia VLPP has an unusual shield decoration. There are perhaps 4-5 different shield designs (horseman, studded, vertical lines, plain (with or without shield boss) and assorted patterns (like this coin) are the main types) with a great variety of differences among each design; but this one is very different. It is also a rarer design...I have had only a few examples over the years and seen a few more. I really don't know what is depicted on the shield-- though I believe it is probably a floral design; but might be an astronomical element or perhaps just a geometric pattern. The crossbar of the helmet also has what looks like a lower case i.
Constantine I
A.D. 319
19x20mm 3.1gm
CONSTANTI-NVS AVG; high crested helmet, cuir., spear across r. shoulder, shield on left arm
VICT • LAETAE PRINC PERP; two Victories stg., facing one another, together holding shield inscribed VOT PR on altar inscribed with the letter I.
In ex. •BSIS•
RIC VII Siscia 84
I found a picture of an Irish shield (O'Donovan's shield) from around the 15th century with a similar pattern