W108 rear seat belt: what is this part?
 

I picked up a 1970 280SEL last year and confused by this part that was flopping around on the lap belts in the back seat. i's chrome/silver plated and says "ALL STAR" and appears to have a "199" stamp on it.

Is it some sort of retractor or tensioner? and if you are familiar with it can you tell me how to use it? It's a clip with a spring inside (however, I have one with no spring and assume it's broken).

I'd really appreciate better understanding what the heck it is (I need a few more) and how to use it. Many thanks, cheers.

Seat belt became mandatory on all US cars in either 1966 or 1967. Or somewhere about that time.

Anyway, one gripe people had right away was the rear belt were always getting lost in the back seat, under the lower cushion. This was not a major problem as no one used them anyway, but it was something folks talked about. As in they had to pay for something that vanished a few days after you picked up the car.

So Mercedes came up with these retractors for the rear seat. In theory they would roll up the buckle part of the belt so it would not slip beneath the cushion. The latch part never slipped away because the latch was too large to fit into the area between the lower and upper rear seat cushions.

I have seen a few of these. I have never found one in working condition. They usually fell apart over the years and folks just tossed them out.

Idle has given a good description. Here are both side pictures of one installed on a partially extended belt as it appears in the rear seat of my 1972 280 SE 4.5. Only 57K miles on the car with the back seat hardly ever used so all the spring retractors still work perfectly
http://i66.tinypic.com/ibdfli.jpg
http://i66.tinypic.com/4rtk4i.jpg
Just noted that you also asked how to use the retractor. Before clipping the belt to the retractor, it is necessary to pre load the internal spring by turning the cylinder several times relative to the wire guides. I believe mine states no more than six turns. The number of pre load turns that you use and the location of the retractor along the length of the strap are all variable and final positions are best arrived at following considerable trial and error. Once you've worked it out with the first retractor, adjustment of the others is much easier.