My first reaction is that there is no reason why Constantine should have a bare bust while Licinius has a laureate bust, but looking at RIC VII for that issue (p. 169) there is a rare Mars fraction with laureate bust for Constantine (# 68, 69), plus there are footnotes for Licinius examples of some of the other types from that issue, so this coin might slot into that group next to the Mars coin for Constantine?