A well tool 1, wherein said central disc portion is domed and is cooperatively positioned within said housing with its concave surface facing said forward end of said mandrel to better withstand pressure forces acting on {HEAD ~ the convex surface} of {TAIL ~ said central disc portion} 
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A well tool 1 , wherein said central disc portion is domed and is cooperatively positioned within said housing with its concave surface facing said forward end of said mandrel to better withstand pressure forces acting on { HEAD ~ the convex surface } of { TAIL ~ said central disc portion }
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