The central surface of the upper side of {TAIL ~ the cutting head} has the form of a convexly curved surface 16, the highest position of which is located at the same level or somewhat below {HEAD ~ the bottom} of the trail 1 a, as best appears in FIG 5 
The central surface of the upper side of { TAIL ~ the cutting head } has the form of a convexly curved surface 16 , the highest position of which is located at the same level or somewhat below { HEAD ~ the bottom } of the trail 1 a , as best appears in FIG 5
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