A few years back I got my hands on a AVO-8. Not in as great an exterior condition as yours but it still seems to measure right.
I solved the problem with the 15V battery by drilling a hole through a piece of wood of a fitting length and putting the right amount of small button cells in to match 15V, and maybe a piece of metal if the number of cells was to small to match the length (I don't remember if it was needed)
If it has any interest I can show what I did.
So here I made a little 11 min. video where i show my simple substitution for the 15V battery. And show the meter in operation afterwards.
You are absolutely right in your status comment, video editing can take a long time. (this one I just did wasn't too bad as it is fairly simple)
Here the links: