Ok its officially Wednesday here in Blighty....
My ammeter has the same seriel number as yours... 5539-233T it came from ex-military stock off a Land-Rover series II which has a 12v -ve earth supply.
The following photos show disassembly and possible locations for an iluminating bulb.
You will need a thin shaft screwdriver and an adjustable wrench.
1) Remove the mounting bar nuts and lock washers and slide the bar free
2) I then removed the nuts on the +ve and -ve connection threads (later I realised you do not need to do this)
3) I then very carefully prised the 3 tabs on the bezel (the black metal rim that holds the glass on) so they were near straight
4) Very carefully rotate the bezel so that the 3 tags line up with the 3 'notches' in the plastic casing
5) Gently pull off the bezel, be careful as the glass is free to drop out
6) Remove the glass disc and rubber 'gasket'
7) Look at the rear of the gauge and you will see 2 small recessed screws, these actually hold the working part of the gauge to the plastic
8) Hold the ammeter upright (you dont want the innards to fall out) and remove both screws
9) Gently place a finger and thumb either side of the needle of the gauge and tip the gauge and the innards will slide free. Do not force it out and remember that the needle is VERY delicate, snap that and its game over
10) I then held the dial and ammeter workings by the rim of the gauge and slowly pulled the ammeter free from the casing, there are wires attached which will only just allow the gauge to be removed.
I took a series of photos for you and from the look of the plastic and the ammeter workings I think the best position for the bulb would be located on the back face of the plastic housing right in the centre. The bulb will not be inserted to far into the hole created and a sutible bulb holder will have to be found/made.
The internal surface of the gauge will be painted matt red and can be done in a disassembled state with a steady hand, working in stages around the wiring. Drilling the rear hole will be fun and will probably be done by hand. You will need to watch out for the internal wires that connect the ammeter to the external terminals. I took a photo of my oil gauge light with the ammeter to give you an idea of scale for the bulb. A thinner bulb might be better, need to look into that.
Also not that when I removed the connector nuts I noticed that the shunt plate had been soldered to the connection screws which prevents any further removal of those items, hence the note in point 2.
I then re-assembled the ammeter....its getting late here and I have done a 13 hour shift and thought it best to quit while I was ahead. When reassembling reverse the process again using exterme caution when replacing the ammeter and also be careful not to get the internal wires trapped in the seating location of the ammeter. It took me 3 or 4 attempts to get it seated correctly without the wires in the way so the small holding screws could be tightened.
I also have a pic of my Lucas 608 mirror which is having a replica mounting boss and stem produced for it. Its important to remove the mirror glass before replacing the stem!. Jay Cushman of Cushman Comp is producing it for me and has supplied me with a few other pieces. The quality of his stuff is superb, I have no kickbacks or affiliation with Jay, I'm just a very happy customer. His web site is below, and his list of replica parts is growing.
GT 40 Parts
The mirror will be fixed to the windscreen glass with clear epoxy.
Hope this info helps.
Andy