the header plate 96 thereby is forced against the fixed lugs 90 which act as abutments that position the header assembly this provides a known location for the header assembly 15 because the surface 101 of the header plate 96 later is used as the locating surface for the header assembly when it is connected to the motor assembly 16 as explained below beneath the clamp assembly 76 are four bell cranks 103 which are pivotal about a horizontal shaft 104 carried by the bifurcated forward vertical wall 105 of the base 75 the bell cranks 103 are spaced apart a distance such that when a header assembly is in the fixture each bell crank is aligned with one of the fixed contacts the upper ends 106 of the bell cranks then are positioned adjacent and immediately inwardly of the fixed contacts 98 from the mounting shaft 104 the bell cranks 103 have elongated end portions which fit through an opening 107 in the base 75 extending beyond the base where their upper ends are engaged by spring biased plungers 108 which are similar to the plunger 91 that biases the clamp 76 to its open position this urges the bell cranks 103.