Sorry, I know this is quite an old topic, but I feel I should post this for completion 😀
When I said reissue before I was referring to the 90s Fender reissue, not the Squier Classic Vibe ones. And a slab body is exactly what you described the '65 Mustang as, i.e. the same thickness all over the body. I think the original Duo-Sonics were just a slab body.
Hope that's cleared things up 😀