I think it was an o-ring. Prolly can do a search here and find it. Seems there was a particular size that best filled the gap.
i believe you are correct,but i am not sure as to the size of the o-rings. you may have to go really small and double up to find the correct size.
someone on the forum has used them to fix their problem, i just can't remember who.
i don't have extensions on my LCP, i just curl the pinky under the bottom for now.
i think the original problem had to deal with making their magizine operate better and not the extensions --i am really not sure though.
you might want to pm jocko to see if he knows the answer, jocko has extensive knowledge with kel-tecs and since the pistols are similar maybe the problem has been addressed on the kel-tec forum in the past.