Where do the fuel injector O-rings go?

I'm replacing the injectors on my 300E (12-valve engine). I ordered the injectors from fastlane and received 3 parts: the injectors, the rubber seals, and mysterious O-Rings. The injectors and rubber seals are easy, but for the life of me I can't figure out where the O-Rings go. They don't seem to be installed anywhere on the old injectors. They don't appear in the W124 shop manual. Does anyone know?

It looks like they would fit nicely in a groove that runs around the rubber seal, but since the old one didn't have them I'm mystified.

Matthew