Mine are also past the nubbie-slots. It's never moved, don't worry
about it. The slots are helpful when you want to take the motor out,
though (which I've done) and replace it (after you're done using it in
the base), as you don't need to completely remove the clamp bar.
The 7518 has all the same type of bases as the 691 (fixed, plunge,
d-grip, etc). I have the fixed (screw-type) base, and the nubbies are
used for depth control when the motor is in that base.