Wiring diagram/photo

[The Spoggles Dad]

Hi, I managed to get a good price on a second hand set from eBay, it includes the battery operated lap counter which unfortunately had, had batteries left in it and they had corroded. I intend to try and get it working by taking the contact out and cleaning them. The father in law has a soldering iron can use but when I took the cover off the back only three contacts had wires on. Can anyone post a photo of there's or a wiring diagram so I can make sure mine is wired correctly. 

 

Obviously I can and will take a photo to show where the wires are currently before I disconnect them. I just need to know if there should be a wire on the fourth contact, there doesn't seem to be a loose wire

 

thanks in advance