No concrete proof that there’s a metal difference between Eppi and Gibby three points, the Greco three point yes, not sure of the others but it wouldn’t surprise me if there was a difference. As far as fit goes, I’ve been under the impression that Gibson and Epiphone three points bridge bodies are interchangeable, just not the studs. Eppi uses metric studs and Gibson uses SAE studs, so neither one will thread into the others insert. Eppi also uses a raised face body insert whereas Gibby uses a flush mount body insert. So as long as you use your original bolts I’d think the Gibson three point body would work on the Eppi.
All that said, before you buy one you might want to give a shout out in this club and the SG club to see if anyone has a spare they don’t want, you might get it for free. I had two of them (both black) that I’d have given you but I can’t find them, if I run across them their yours.
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