Farberware teapot handle removal

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I got an older Farberware teapot from a yard sale at one of my neighbors. I've got it very clean except for the handle and the part it attaches to. I would like to remove this handle to clean it better. There is a recessed area in the back of the handle with some type of screw or fastener. This appears to have a slot in addition to an indented area that is straight on two sides (longer) and curved on the others (shorter). It reminds me of the type of screw that was used to attach paneling in mobile homes (don't know the name of this kind). I tried using a regular straight blade screwdriver (a good one), but it seemed to slip easily, so I stopped as I was afraid of stripping it out. Anyone know how to take it off?

 

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When in Menard's the other day I looked at screwdrivers, and found that shape is called a "clutch drive G" screw. I bought the driver (made in USA), and tried it the other night. I thought it was the right size, but it seemed to slip, so messed up the screw head a little. I stopped so as not to make it any worse. May need the next larger size driver. The screw doesn't appear corroded in any way, so don't know why it won't come out.

Link is to pic of this style teapot.

http://s.ecrater.com/stores/51368/4f5b9c0788493_51368n.jpg