I noticed in my 955TT that the 2 outside rear seatbelts seem to wrap around a big polished metal buckle type thing. It appears to be a permanent fixture. Anyone know what they are??

 

Those are affectionately known as the "Cowboy Belt Buckles" and were installed by Porsche on a TSB due to a recall on the rear seatbelts.

 

From the TSB:
"Porsche has found that certain vehicles may not fully comply with a part of Federal Motor Vehicle Safety Standard (FMVSS) 208 regarding the use of the seat belt locking feature. This locking feature is used when installing a child seat in the rear outboard seating positions. A small number of non-LATCH equipped child seats cannot be installed as required by FMVSS 208, because they have a very short belt routing path. Depending on the child car seat used, it is possible that the outer rear three-point belts do not meet the US specification with regard to securing child car seats. To comply with this US specification, an additional belt strap holder must be installed on the two outer rear three-point belts on affected vehicles."
Essentially, the inertia lock doesn't work unless the belt is pulled out a certain distance. The belt strap holders (Cowboy Belt Buckles) take up some of that slack so that the inertia lock will still work with child seats.

 

You let people sit in the back seat.......?