Can someone explain what a Nub trap is?

Ive not heard that term before. Thanks.

http://www.ebay.com/itm/252379005612?ssPageName=STRK%3AMEBIDX%3AIT&fromMakeTrack=true

The nub is the pin that goes through the frame of the trap to keep the chain from sliding down the frame. You see them on government traps.

Thanks cascade!