Abstract: A hollow closed housing is provided to be interposed in a well casing string and the housing includes vertically spaced upper and lower walls interconnected by first and second pairs of opposite side walls. The upper and lower walls have registered passages formed therethrough relative to which axially spaced lower and upper ends of upper and lower well casing sections may be sealingly secured and a pair of superposed control plates are slidingly mounted within the housing between the upper and lower walls thereof for inverse shifting toward and away from one pair of opposite side walls of the housing between the second pair of housing side walls. The plates have oppositely facing elongated keyhole-shaped opening formed therethrough including large diameter partial cylindrical first ends and small diameter partial cylindrical second ends opening into the first ends remote from the closed ends thereof.