Oh, another idea from my sleep-addled brain and from my list for Groundwave:
Part of me has doubts that the M-4 could have been the only test bed for what became the M-5. After all, while they’re both WarShips, they’re not terribly similar.
The Lola I, II and III, on the other hand, use basically the same space frames as the M-5, while possessing weaker firepower than the later M-5, making it more able to be brought to heel during testing phases. Once all the bugs were worked out on the refit Lola I/II/III hulls, then, they could go for the big production runs of the much more powerful M-5 series.
Lola’s also have the benefit of having been in production since the dawn of the WarShip age, so parts are probably scattered everywhere, just like derelict hulls. I imagine they’d be easy to get ahold of, comparatively.