What is the 7th access on my leisure battery?

Hi everyone!   Oh dear -- I am so ignorant.   
My leisure battery has 6 cells, black covers, with the usual big cross-head grooves. Then below them, still on the top, is another one just the same, but with transparent top. What is that? Is it just another ordinary cell? Or something else? I don't want to top it up if I shouldn't!  
Please enlighten...
Many thanks!  
Jinny

Usually its a clear 'window' which when viewed from directly above shows green when charged or red when needs charging. It doesnt unscrew.