Ah. So they use the same part, and just turn it on its head for the combo version?
Yup. I don't know the exact technical specifics, but your 40C is probably mostly the same as a 100 w head, just flipped upside down with two less power tubes and maybe smaller transformers.
Back when Marshall made pretty much only 50 and 100 watt amps, the heads and combos were literally the exact same amp design. They were heads flipped upside down into combos.