Does the plate in the middle of the back under what must be directly between the pickups have anything to do with the Evertune? Because I can't think of a reason for it to be there unless something bad happened.
You mean the end of the routing which is closest to the neck? It sure does, holds the whole thing in place! The other metal plate is what they call the comb. That is what keeps the 6 (or 7) individual saddle units parallel to each other. And you have NONE of the intonation and tuning issues with it when changing strings. There is no easier and faster bridge on the market to change string on. Once you tune it, changing a whole set of strings is 3 minutes top. For starters, you don't tune with the tuning pegs, it is done with a screw on the saddle itself, but you don't have to use them if it was previously tuned. When you change strings, you just thread the new string through the saddle, make a few turns on the peg and it automatically sits back into the middle zone where it produces the constant tension. The only other time you would use the tuning peg is when you want to alter the saddle's behavior (ignore or rreact to tension changes). Keep in mind: this is not a trem/vibrato. It is a fixed bridge, it's only movement is in order to keep the tension constant on the string, thus achieving unaltered tuning and intonation under any circumstance. If it is properly in the middle zone, you can give your tuning pegs 4-5 turns and nothing changes! It has some devilish geometry and a spring which does all the "magic". If you turn the peghead enough, you can deck out the moving part of the saddle and then it's no different than any regular fixed bridge solution in terms of behavior.
Here's plenty of info:
https://www.evertune.com/resources/maintenance.php
JB is right by the way: extremely expensive. I had to change a single saddle on mine (I bought the guitar used the previous owner broke the tuning screw), set me back around 100 USD... Basically you can buy a mid tier guitar for the price of a single bridge. In my experience it works as advertised, but as Goob said it requires a huge chunk of wood removed and a bunch of metal goes in, so alters the tone. I've no ide how much, never a guitar to ET to test the effect. Like I said, a very handy tool to have around, but I am still more comfortable with a Floyd under my hand. It might just be the 15+ years of headstart.
EDIT: I think I know which plate you mean, zoomed into the picture! The ETs I saw (mine included) Have a one piece backplate. This one seems to have a two piece backplate, one giving access to thread the strings, the other gives acces to the metal plate in the cavity which hold the bridge in place, so I guess that rectangular plate is there to allow service access. Never saw a solution like that though. Maybe the guy who installed it tried to save some wood? If yes, than I say its a smart move!
EDIT 2: Its completely official solution, it looks like its a G model intended for Gibson and Gibson style guitars that have this 2 piece backplate.
https://www.evertune.com/shop/_img/b...ckplate__G.jpg