Yes, I think it's the case that the "vintage" style tuners are mated to a thicker headstock (SC245, McCarty, et al, have that feature) which would make the installation of Phase II or Phase III tuners an impossibility. PRS really should develop a retrofit locking tuner that fits within the constraints of that design; I'd certainly rather buy a PRS OEM product than something made by another company, if I could. I can't say I'm a fan of the Kluson tuners that are used, as they seem cheap and more prone to slippage. Perhaps I'm spoiled, though; I'm a massive fan of the Phase IIIs which give me zero problems.
W.