Since it's an E320, he must be talking about a late 124 or 210 car. I've had one of each so far.

You're not missing anything important. The plastic receptical (as you call it) is for holding the latch when the seatback is folded down. When new, there were some rubber band type contraptions which held the latch in that receptical. The latch had to be pulled out to buckle the belt, and would snap back when the belt was unbuckled. The third seat has the same setup.

Over time these things seem to have given up the ghost - likely the rubber fatigued and broke. I never looked into replacing them - they are strictly for convenience and appealing to the German engineers obsessive compulsive syndrome. Dealer might be able to get them if you wish.

- JimY