How to wire a ceiling rose?
Often, electricians used ordinary black/red/green twin&amp;earth for C but marked the black wire with red tape (to indicate "switched live")

You seem to have something nonstandard, perhaps a spur (therefore missing B) without an earth to the switch? You'll have to use appropriate methods to determine what is what.

See answer to similar old question https://diy.stackexchange.com/a/32251/2815