Which then raises the question of why the front line units didn't have enough mechs in the first place. These machines appeared from the beginning of the invasion, before any of the clans had suffered significant losses. If they were a post tukayyid thing, then sure, but they weren't. Either way, it creates the impression that somebody forgot to pack enough mechs.
The idea that they were refits produced in the homeworlds to bulk up second line numbers for the invasion, rather than being retrofitted from captured hardware during the invasion, makes more sense to my brain. And it seems to fit with the master unit list giving several of the machines to the home clans as well.
Because they were denied bringing in more than front line troops initially.
Remember, they Khan's had to petition to be allowed to bring in PGC's after they started taking losses & couldn't use FL troops for garrison.
The captured IS crap was isorla essentially.
So they gave it a quick weapon swap with what was on hand & put it back into action.
A quick fix to be used in the early waves (Twycross) before reinforcements could arrive from the homeworlds.
Hell, the Jags were so in need of mechs that they shipped something like 2 galaxies of salvage from DC OZ back to the homeworlds to be refitted & used back home for garrison.