Checked ETKA

These are the only #'s listed in the diagram
037906031P Entire Injector
035906149A O-ring
037133047 Spring Clamp
037133555A Injector end
037906149 Some unknown injector Support washer

It doesn't even look like seal kits come with any
Parts4vws.com and Parts4Audis.com - Injector Seal Kit Digifant

I remember when I did my injector seals for CIS-e I ran into a similar problem with no one knowing about the E-clips that held the o-ring in place. I was told not to worry about it as it is more of a vacuum system than a pressure system, so the o-ring would still seal. It could be different for Digi. If worse comes to worse I'd imagine getting some generic E or C-clips should do the trick.