So... the "C" refits (such as the Archer C, Marauder C, Rifleman C, and the like) have been with us since the Twycross scenario pack. At the time, they were described as refits of captured inner sphere machines.
(full disclosure, this explanation never sat well with me. It was early in the invasion before the clans had taken significant losses, so it created the impression that somebody had forgotten to pack enough mechs. But I digress).
Their entries in the master unit list create a more complicated picture, however. Most of these variants are also available to certain home clans during the invasion (in some cases the only invading clan that gets them is the Steel Vipers). Most are also available to the Inner Sphere powers bordering the invasion (a couple are available ONLY to those inner sphere powers). I could provide more details, but the Master Unit list is apparently down at the moment. Only the Victor C appears to match the original narrative, appearing only among the Jade Falcons and quickly going extinct.
Anyway, to get to the actual question. Is the original idea that these mechs were refits of captured inner sphere machines still the full story? Or perhaps is it more complicated? With some being pulled out of brian caches to bulk up forces for the invasion, others being recovered machines refitted to replace losses, and still more actually being refitted by inner sphere forces from captured clan salvage?
Or maybe I shouldn't stress over it because the "C" mechs were originally just placeholders because the writers hadn't yet gotten around to making any actual second line mechs. :P