I know just plain old Constantine coins to some but I love them. I just picked up my second one of these this time with the S on the altar. The one I had. Constantine I. 307/310-337 AD. Æ Follis Obv: CONSTAN-TINVS AVG, two eight-pointed stars and row of three dots on a high crested helmet and cuirassed bust left, holding shield and spear. Rev: VICTORIAE LAETAE PRINC PERP, two Victories standing facing one another, holding vota shield altar below with I, exurge A SIS(star). RIC VII 51 (19mm, 2.93 grams Siscia mint, The one I just picked up. CONSTANTINE I THE GREAT (307/10-337). Follis. Siscia. Obv: CONSTANTINVS AVG. Helmeted and cuirassed bust left, holding spear and shield. Rev: VICT LAETAE PRINC PERP /∙Δ SIS∙. Two Victories standing facing one another, holding shield inscribed VOT / PR over altar between them. RIC VII 84. Weight: 2.83 g. Diameter: 19 mm.
Your second coin, (like gogili1977's first) has a cross on the helmet. That adds additional interest for many collectors. Mine just has a star: Siscia mint, A.D. 319-320 RIC 95, variant Obv: IMP CONSTANT-INVS AVG Rev: VICT[dot] LAETAE PRINC PERP - Two Victories, facing each other, holding shield inscribed VOT/PR over altar ΓSIS[star] in exergue; S in altar. 20 x 18 mm, 3.2 g. (Note: This coin has the obverse inscription of RIC 101, albeit with an unrecorded inscription break, and the mint mark of RIC 95. RIC 101 is part of a group of coins labeled "Irregular." One possible explanation is that these "irregular" coins are part of the same series as RIC 93 - 99.) And here's its barbarous cousin:
