In addition, regarding the backplate near consummate comments, there are many changes even between versions with the same Toroid version. Over the years, a few version notes:
1) Backplates with attachment screws exposed (original, brown ones, very old)
2) Backplates with screws hidden, a single low, jumper/banana tone control, vs. the later versions with 3 tone controls (quite old).
3) Backplates where the bass transformer was wired to repel dust instead of attract it (forgive me for lack of more technical explanation on this one)
4) Backplates with the 3 tone controls and new, improved mixer box and new values on the “crossover” components. I believe only the later versions of the 3 tone control backplates had the mixer and component value changes.
I think there were other component changes above the few I mentioned above. Some of the backplates may have had these updates incorporated inside, it is hard to know without detailed photos of the underside. I wouldn’t be afraid to send them into SL to be completely updated.