Only slots for vacancies are 77FM & 75FM. So a very senior 75 FO only option is to move to 77FM (114 vacancies). But if 114 pilots senior to that pilot (regardless of subject to assignment) bid it then there are no vacancies.

The only pilots that can “bump” junior pilots are those in seats that are in excess of max staffing level. This prevents the bump and flush of 4.A.2.b days! There were pilots bumped out of Captain slots in seats that were not overmanned by pilots subject to assignment. The now bumped captain is suddenly subject to assignment and can bump a 75 Captain.

This occurred 684 times (includes both captain and FOs). That is how many pilots were subject to assignment. So only that number of pilots could “bump” a junior pilot.