Click for large image
Manufacturer: Fender
STRING ACCESS
Most of the backplates we sell have the modern slot in them. The very first Strats used plates with six small round holes in them, but these are inconvenient to use, as the holes in the trem block don't always align with the holes in the plate, making it awkward to get the ball-ends of the strings out. On the vintage-type plates, Fender's solution to this is to use six elongated holes, which will typically line up better with the holes in the trem block. In modern Strats, they've gone a step further by eliminating the holes altogether and replacing them with a single long slot, which is even more convenient to use.
Those backplates with no holes are designed for guitars with Floyd Rose tremolos, where the strings load at the top of the bridge, rather than passing through the block from the rear.
TORTOISE/MOTO BACKPLATES
The top ply of these products is sometimes made of celluloid, and consequently it can be unstable. Solvents may be released from the top ply over time, which allows it to shrink slightly while the underlying plies remain dimensionally stable. The net result of this is that the material cups (like a very shallow bowl), so that when placed on a flat surface its edges may be raised above the surface. If you're familiar with vintage Fender guitars then you've seen dimensionally-unstable pickguards - early '60s Strats will often have a crack or two in their pickguards that run from a perimeter screw hole out to the edge of the pickguard, for instance. And if you've ever looked at an old Jazzmaster with a tortoise pickguard and noticed that the wood screws are going in at angles rather than straight, this is the reason: The pickguard has shrunk, while the wood has not, so the screw holes no longer line up perfectly.
Even if the backplate is cupped when it comes out of the package, it can be screwed flat onto the guitar (the perimeter screws will easily pull the edges of the cupped plate down to the body), but even with the perimeter of the plate held to the body, the plate as a whole will never lie quite as flat as a plate made from vinyl, or some other modern, dimensionally-stable material. This comes with the turf though, if you want the look then you have to put up with the shortcomings of the material.