Here's a paragraph that explains what each engine type is and what car they were originally fitted into. Your YBB is a Red Top engine, but exactly what year of manufacture it's difficult to tell unless you know the engine number:

The various colour cam covers that distinguished each version were as follows: Red: YBB (Sierra Cosworth 2wd, both 3-door and Sapphire), YBD (Sierra RS500), YBJ (Sierra Sapphire 4wd, non cat); Green: YBG (catalyst equipped 4x4 Sierra Sapphire Cosworth); Blue: YBT (large-turbo Escort Cosworth); Silver: YBP (small-turbo Escort Cosworth).

Lifted from - https://en.wikipedia.org/wiki/Cosworth#The_YB_series