I've never seen a Birdy in person, so I'm just guessing here.
- What you are showing is the bottom of the "stem" (handlebars are to the right out of the picture) which fits into the top of the fork or steering tube. Is this correct?
- Or does the shaft (with arrow) fit through the headtube of the frame, and then does the fork assembly clamp to the end on the left, which is effectively the bottom of the steering column?
I'm guessing my second guess is the correct one. That the Birdy uses a reverse approach from most modern traditional road and MTBs, that the stem holds the fork in place. In this case the fork holds the stem/bars in place.
If this is the case, are you certain the "seal" (black ring on left) is for the upper bearing race (black ring on right)? My guess is that it would fit at the lower end between the head tube and the lower bearing race and just above where the fork assembly clamps to the stem/bar tube. None of the pictures I've found on the
Birdy Website depict the area where the fork clamps to the stem/bar assembly. Have you thought about contacting them?