What is this kind of screwdriver called?

I am trying to take a broken coffee maker apart, and in the base there are four screws; the head of each screw looks like this:
|S| |L| xxx xxx xxx |O| |T|
Where xxx represents where the slot isn't cut through, and | | represents the slot. You'd need a standard slotted screwdriver with a sort of a nibble taken out of the middle of the slot. What's that kind of screwdriver called?
Thanks Liam
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I've found it here:
http://www.thefastenerconnection.com/Products/TamperResistant.html
It is the "clutch 'A' " type of tamper-resistant screw. Now, to find the tool. Liam
I wrote:

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check harbor freight if there is a store near you - they have a cheap kit of security screw bits. If they are round head screws I have used small vise grips in the past.
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