C13/1 came on some Lightnings as did C1/1 the lightened clutch drum. View attachment 39033
I have a lightened drum like that. Seems to me that they were used on B’s (with the normal plate), but they stopped drilling holes in them at some point.
I wondered if my C13 was actually a C13/1 because it came in a bin that included couple of real Lightning parts. I see the hole pattern is different, wonder if they were all drilled the same or depended on the mood of the worker? Either way, I think mine is prettier than yours. Whoever did it, certainly gave it some thought.