Wow! Nice Simpson 260! Take it apart and get the batteries out if you haven't already. Even if they have corroded up, you can usually clean them up. The original circuit diagram for the 260 is available, and I think Simpson still "refreshes" the 260. There are parts available. Here is a cool website about the Simpson 260. It takes several minutes to load the web page but it is pretty cool if you wait for it. A bit of electrical nostalgia there. I cant wait till LarrBeard hops in about the other meters in your pictures.

http://simpson260.com/