Hi John:
Yes, the C-20 lower control arm is different from the C-10. They may LOOK the same but they have different attachment to the subframe. They are not interchangable. I am surprised you cannot find one for a C-10...they should be more common.
FYI: despite C-10s and C-20s looking similar, you would be surprised at how little commonality of parts they have concerning suspension.
Regards,
Henry