Field Report: 2765 Periphery includes Random Assignment Tables, a BattleMech one for all the major periphery factions and a generic periphery one for vehicles and aerospace.
We know most of the personnel came from the major periphery powers, where this was seen as a war of liberation from the Star League.
But Liberation of Terra I also tells us that Amaris used shell companies to purchase from greedy manufacturers all over the Inner Sphere. So it sounds like there is quite a wide range of equipment in the Secret Army. It says a lot of the tanks and battlemechs in the secret army first saw action in the Age of War, but the units were stiffened with a few newer models/designs.
They also made heavy use of combined arms formations, mechs and armor for offense, with infantry-centric defensive formations.
EDIT: I think on the MUL I'd look at Production Era: Age of War. Based on what I've read. Maybe to the tune of 3 out of every 4 units coming from that. With the last one potentially pulling from something newer, Great House or Inner Sphere General design from the Star League era.
