I'd definitely say late Mastery epoch, on the transition to Core epoch.
The heavy armor and defensive ballistic weapons come from early mastery (Dominator etc.), the inefficient rotating shields from the middle Mastery epoch (Enforcer, Lasher) and the (at the time probably still primitive) energy weapons are a novelty of the late Mastery epoch.
I'd also speculate that the Venture and Mule are not military designs, but armed civilian ships. Private corporations were probably quicker to adopt the unproven energy weapons. The military, hesitantly, used offensive energy weapons for the first time as a very small part of the armament of the Conquest-class, likely the first ship of the Core epoch.
There are some more transitional ships in the game, like the Sunder and the Apogee.