This appears to be a spacer that takes up the slack between the brake pedal lever and the end of the booster itself; the end that is inside the car.

The 'Catalog of Illustrations' that I have for the 109 shows this part as being nothing but a spacer since the booster is farther away from the driver in a 109 than in a 220/8.

It could be that your car will not require this spacer, but if it does then you should be able to use your old one.