Some lead-acid batteries have two extra covers behind the terminals (See the images below). I understand that the 6 holes in the row in front of the terminals are used to water the cells. But what is the purpose of the other two, I've open them and they have only some plastic. What are for then?
I think the purpose of these "covers" is to allow the same plastic case to be used with batteries that have different kinds of connection posts. I do not think they have any maintenance or electrical purpose.