The early PTK like the one pictured in my original reply has no limit valve, depending on the axle ratio the PTK can occur at some pretty high road speeds and this can be annoying. My '57 NY with 2.94 gears will still PTK at 60 mph.

The '71 and later PTK module contains a limit valve and spring to limit the PTK to below 40 mph but the limit function won't work when retrofitting the '71 and later module to the earlier VB. The only way to get the limit feature in an earlier trans is to swap in the entire VB.