This coin is just like RIC 73 except for the altar. Starting with RIC 73, there is an I on the altar for off. A and B; and an S on the altar for off. gamma,delta, and epsilon. Well there should be an "S" on the altar of this coin, but it only has two diagonal lines like a large X.
The reverse on this coin looks like earlier issues (RIC 47- 56) in that the altar is very similar as are the victories, which are shorter and more squat than the usual RIC 73. An older reverse die may have been used with only a bit of reworking to make the mintmark correct.
the first coin below is the topic coin and the second picture is a normal example.
Constantine I
A.D. 319
20mm 4.1gm
IMP CONSTANTINVS PF AVG, laureate helmet and cuirassed bust right.
VICTORIAE LAETAE PRINC PERP [ Joyous (well-earned) victory to the eternal Prince] two Victories stg., facing one another, together holding shield inscribed VOT PR [VOTA POPULI ROMANI (vows of the Roman people)]on altar.
In exergue gamma SIS dot
cf. RIC VII Siscia 73
