I think of custom guns like custom cars, where the level of customization is graded based on the amount and type of work done. A simplified grading system might look like this-
Production cars are production cars. Sure, you can choose engines, trim, wheels, colors, interior material, etc, and a high end car will get more attention on the assembly line than your average Ford or Chevy, but it isn't custom.
Start changing wheels & tires, adding pipes, bolt on engine mods, pinstriping, a simple respray, lower it, jack it up, etc. and you've upgraded/personalized the vehicle but don't really change it from it's original configuration and this might be considered hot rodded/tuned or maybe up to a mild custom.
Take the same car to hot rod shop (or do it yourself if you can) and chop it, channel it, french the lights, bag it or a 4 link or other suspension mods, swap in a different engine/trans or build a blown or turboed motor for it, custom color or multi color paint, fully redone interior and now you've radically altered it from it's original configuration requiring body/metal work, etc. That's a fully customized car.
Now, equate that to handguns and 1911s in particular-
Production guns are just that. You're limited to certain models, lengths, calibers, maybe there's optional sights or finish or, as with Baer, an accuracy guarantee, but even if it's a high end production gun and it gets more attention on the assembly line, you can't have changes made beyond the few options they offer and the next buyer can go to his LGS or the manufacturer's website or another guns seller and get exactly the same thing out of the catalog, that isn't custom.
Take whatever gun you have and change grips, sights, do a spray-on refinish, maybe swap some small parts, do an action job or reliability work, etc. but don't really change it from it's original configuration and now it's upgraded/personalized/hot rodded maybe up to the level of a mild or semi custom.
Now, take that same gun to a skilled gunsmith or have him source a frame and slide, then collaborate with him on everything you want it to be (or do it yourself if you can) - melt it or carry bevel it, add checkering, chain links, serrations, ball cuts, etc, upgrade the internal components, hand fit everything, surface prep to the desired level of polish, any sight combination, a permanent finish of your choosing, and so on - combining your vision and his talent/artistry to radically change the gun from original, requiring metalwork, fabrication, etc., resulting in a unique one off piece. That is a fully customized gun.
That's my take on it. anyway. Obviously, there's lots of room for interpretation but I'm sure you guys get where I'm coming from.