Although I've never seen any documentation that explains it I have noticed that they are only on 340 cars. Up until recently I could not see anything unique about them, however after checking out a bunch of K-members comparing 340 to 318 cars (the 318 cars didn't have the slugs) I think I found out what they were for. The 340 units had a reinforcement piece which looks like a half of a large washer welded to the rear of the LCA pivot hole, likely due to the theory that 340 cars might be getting a bit more abuse. The slug was likely added and used as a quick identifier for workers when installing a K-member into 340 car since the reinforcement piece is not very obvious.

First pic here is a 340 and a 318 K-member (both are from 1971 Dusters)