In bathrooms and toilets - extractor fans are often controlled from the light switch - switching on causes the fan to operate and a separate power feed allows it to over run when the light is turned off.
The extractor then works automatically as the light is turned on.
If they're capable of pull-cord operation it should be quite obvious where to fit the cord - a switch with a lever and a hole in it to attach the cord.
You say that both of them work - then why should you need a pull cord?
The slot in the case might well be to adjust the period of over run - but that's just a guess.