3 is a dog 9 overhanging longitudinally from the blade toward the rear of the latter and having a circular arc profile shape substantially mating the profile shape of the recess 8 into which it is adapted to engage in press fit relationship as explained hereinafter the support 2 is provided at its heel end with a seat 10 which opens both into the groove 7 and inwardly of the hollow heel portion 5 of said support said seat 10 also has a width dimension adapted to receive in press fit relationship a trailing portion 3c of the blade 3 defined on this trailing portion 3c is a dog 11 jutting out toward the toe portion of the blade 3 and having a semicircular profile shape a plate like lever 12 is housed within the heel portion 5 of the support 2 being pivoted therein about a pivot pin 13 which is carried on said portion and extends across the seat 10 the top end of said lever 12 is journalled on a peg 14 carried on the head 15 of a screw 16 which is supported rotatably on the heel portion 5 the screw 16 can be rotated by means of a knurled wheel 17.