Some climbing spikes (gaffs, spurs) have a wide shank, this restricts the operation of the locking button. Removing the button and replacing with a rope retainer will change the way the climbing line is held in the device. The installation of the rope retainer also requires a longer axle bolt, this is supplied as part of the unit.
If your old unit is still operational and you have upgraded your climbing spikes, however the locking button is now impeded by the shank, this is the solution.