FAA proposes to fine United
 

https://www.faa.gov/newsroom/faa-pro...ng-maintenance
In the 80s my employer modified the 747 procedures which the FAA did not approve with the result as the N registered aircraft could not operate into the USA as illegal although the HB reg. aircraft continued until the dispute was settled.

You got my interest. Care to tell a little more ? Looking at the fleet history the Zürich and Geneva birds were re-registered from N- to HB- on Sept 1992 as -IGE and -IGF could that be in relation with what you hinted above in your post ?