As well as the relatively common issue of the plain MHTA mintmark of 3-turret campgates, for both Augusti and Caesars at Heraclea (16 - 20), RIC VII highlights a myriad of (generally much-scarcer) variations of dots in this MHTA issue, including dot before (21 - Licinius II), dot after (22 - Licinius II) , dots before and after (23-26 - Crispus, Licinius II and Constantine II) and dot in right field (27 - Licinius II). The introduction mentions this "... but the varieties of the MHTA mark exclusively comprise scarce obverses of the Caesars. It is possible that Heraclea at that stage employed a series of double marks, but the scarcity of the material possibly does not give a true picture of the coinage, i.e. obverses of the Augusti could have been struck in all marks, though no coins have been preserved" (until now!).
While I don't think it has any implications for the dating / ordering of these early issues struck in the aftermath of the first civil war, interesting nonetheless!
