thus is urged distally and downwardly causing a rightward front pin 298 to drop into distally open step structure 299 formed in the right half shell 156 which is urged into this engagement by a compression spring 300 hooked to a leftward hook 301 on the anti backup release lever 248 between the rightward front pin 298 and the longitudinally elongate aperture 294 the other end of the compression spring 300 is attached to a hook 302 formed in the right half shell 156 in a more proximal and lower position just above the closure yoke 162 the compression spring 300 thus pulls the distal end 254 of the anti backup release lever 248 down and aft which results in the rightward front pin 298 locking into the distally open step structure 299 when distally advanced once tripped the anti backup release lever 248 remains forward holding the anti backup plate 266 perpendicularly thus allowing the linked rack 200 to be retracted when the closure yoke 162 is subsequently retracted when unclamping the end effector 12 an upwardly projecting reset tang 303 on the closure yoke 162 contacts a bottom distal cam 305 of the anti backup release lever 248 lifting the rightward front pin 298 out of the distally open step structure 299 so that the anti backup compression spring 264 can proximally push the anti backup cam tube 268 and the anti backup release lever 248 to their retracted positions the handle 20 especially the linked rack firing mechanism 150 is described in greater detail in the afore mentioned u s patent application ser no 11 052 387 already incorporated by reference such as depictions of a side pawl assembly intermittently driving a linked rack in fig6 however a side pawl assembly 285 is modified.