Wheel Hop

Slapper Bars worked well on the mono-leaf rear spring cars where the front half of the spring would "wrap" - literally twist into an "S" and snap back straight causing the wheel to hop, again, and again.
Slapper Bars created a solid link to the front spring eye to the shock plate, preventing spring distortion
Preload could be adjusted by rubber bumper .
Calv took that geometry and made it shiny and charges a fortune for it .

This wasn't nec with the mopar multi-leaf, short segmented springs, as explained in the Mopar Chassis manual.
Those that read the manual, learned that clamped front segments did what the slappers did, a solid link to the front spring eye, preventing "spring wrap" .
The adjustable pinion snubber controlled the torque, and weight transfer to the rear axle, - planting it .
So Mopar guys that know, point and giggle at slappers on a Mopar, cuz the owner and chebby guys don't know !
This doesn't mean slappers won't work on a Mopar, just unnecessary, - if you know .
That Chassis manual was our Bible back then, launching the Max's !