[Comartemis]

Something like that.

The only Ancient Belka users we've seen in action (as in "in pitched combat", not standing around casting support magic like Shamal) are Zest, Signum, and Vita, and every magic attack that we've seen from these guys are either of the physical attack kind (melee and solid matter attacks like Vita's Swallow Fliers) or make use of a mana conversion affinity (Signum's and Agito's flame techniques). This says to me (keeping in mind the limited amount of data we have to draw from) that the Ancient Belka system makes use of solid pseudomatter rounds and physical damage as opposed to the Mid system's emphasis on magic damage and energy-based attacks.

The Modern Belka system, as a hybrid of these two systems, makes use of the Belkan focus on melee combat while using the Mid system's energy attacks, which are more suited for the TSAB's apparent preferences for knock-outs instead of kills in combat.

This is only a personal theory of mine and has no official basis in canon.