10 the gate leaves 16 a 16 b are typically constructed of a suitable metal as best seen in fig3 and 5 the front of the face block 14 comprises first and second surfaces 14 a and 14 b angled rearwardly and upwardly and rearwardly and downwardly respectively from a central transverse separating edge the rectangular gate leaves 16 a 16 b are mounted on and ride on the angled surfaces 14 a and 14 b of the face block 14 the gate leaves 16 a 16 b are each capable of independent movement in upwardly angled and downwardly angled directions respectively along the corresponding surfaces of the face block 14 a and 14 b as best seen in fig2 the gate leaves 16 a 16 b are held in place on the respective surfaces of the face block 14 by respective sets of gate guides 20 a 20 b and 20 c 20 d thus gate leaf 16 a is retained by gate guides 20 a and 20 c and gate leaf 16 b is retained by gate guides 20 b and 20 d respectively turning to the operation of the gate leaves 16 a 16 b and considering gate leaf 16 a and corresponding gate guides 20 a and 20 c as representative while the gate guides 20 a 20 c retain the gate leaf 16 a in place on the surface 14 a of the face block 14 and prevent lateral movement thereof the gate leaf 16 a can move axially through the gate guides 20 a 20 c along the surface 14 a of the face block.