wherein said front face portions and said rear face portions form substantially equally sized and configurated but oppositely directed curved face portions with bearing face radii around said first and second bearing face axes which are distanced in {HEAD ~ opposite directions} from {TAIL ~ the vertical medial plane} through the respective piston shoe, which are substantially equally distanced from the respective portion of said guide face and which are parallel to said first axis but distanced therefrom, and , 
from
wherein said front face portions and said rear face portions form substantially equally sized and configurated but oppositely directed curved face portions with bearing face radii around said first and second bearing face axes which are distanced in { HEAD ~ opposite directions } from { TAIL ~ the vertical medial plane } through the respective piston shoe , which are substantially equally distanced from the respective portion of said guide face and which are parallel to said first axis but distanced therefrom , and ,
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