122 and 124 in one embodiment the webbing frame 120 can be further supported by gussets 136 the cross rails 126 can be positioned to define protruding portions 138 142 at the ends 128 and 132 of the forward and rearward spring rails 122 and 124 respectively that protrude laterally beyond the cross rails 126 the webbing assembly 36 can further comprise a central girder 144 that extends between the forward and rearward spring rails 122 and 124 and is substantially parallel to the cross rails 126 in one embodiment the central girder 144 comprises a forward block 146 and a rearward block 148 connected by a cross member 152 in one embodiment the forward and rearward blocks 146 and 148 are mounted to the same side of the cross member 152 the forward and rearward blocks 146 and 148 can each include a respective notch 156 and 158 defined thereon the forward block 146 can further include a registration block 162 that is mounted flush with the notch 146 in one embodiment the registration block 162 includes an extended portion 164 that extends beyond the forward block 146 in the forward direction in one embodiment the rearward block 148 includes an extended portion 166 that extends rearward of the notch 158 formed.