There is an article from the PreWar A7 Club detailing the process to repair an ammeter from a 1932 A7.  Below are details for a Ruby ammeter which is very similar. The first point to note is if the ammeter does not move or does not show any amps before taking it apart, try a bit of heat (airing cupboard) and/or some WD40, it may just be stuck, worked for me.

There are two main types: loop-through connection or terminal, although I have seen a number of different part numbers on the terminal type. If the amps appear to go the wrong way, try swapping the wires over, or in the case of the loop through, loop the other way round. To remove the bezel on the loop through, you will need to bend the case as it is a press fit (see photo).

On the terminal type, this is similar to the 1932 model (see another article) and you will need to apply heat to melt the solder (see photo). My car came with a terminal type that someone had taken apart already and had broken the needle, so be careful when you take the bezel off, as the face can come off at the same time and catch the needle. Not much inside a bent piece of wire, needle base and a magnet around the outside.
If you are really unlucky, the gauge may look like the one in the photo which has suffered from water damage with the magnet rotting away.
Douglas Alderson DA7C