Mine never had the rubber bung and being the older version the cable adjuster is up by the lever rather than by the collar, unless the cable is the wrong way around ?
what i have found is that if I get to the point where it needs a clean out / service and stops working properly i can just wind in the cable housing on the collar so the nipple (phnaaaar) is so far into the post it cant possibly pop out and it makes my seat locked into position until i get time to service it, you don't seem to have that luxury with your black collar so will need to be more on the ball with servicing i guess ?
you can see the cable housing and collar here:
if you wind it all the way as shown in the pic the cable will never release the pin as it wont move enough, i wound mine in when i first got it, its why i thought it was broken, but i needed to wind it out until it worked smoothly, its a fine line between the pin holding the post up and it dropping though :/