I have no idea what that part is, since I have no knowledge of the lens design details.
However, I have seen similar parts used in other applications as shims on a screw to adjust a clearance or dimension. IMO, such a use is not good design as the shim might drop out if the screw loosens. A precision washer would be much better in such an application, but a horseshoe shim does save assembly time during manufacture, since it is not necessary to completely remove the screw to make a change.
As I said above, I really do not know the purpose of this part, but if I were to be forced to guess I would vote for its usage as a shim.
With best wishes,
- Tom -