The LA9FK4H4H is a TeSys F mechanical interlock — a bar assembly that prevents two contactors from closing simultaneously. It mounts in a 3-vertical stack, physically blocking the armature of one contactor when the other is engaged. That makes it the part you spec for reversing starters or any two-speed motor circuit where simultaneous closure would create a phase-to-phase short. It interlocks bottom and middle contactors in the LC1F265 through LC1F330 range (3 or 4-pole), and the top position accepts an LC1F500 (2, 3, or 4-pole) or CR1F500 (3 or 4-pole). That covers the common F-frame sizes for motor starters up to about 330 A in AC-3 duty.
What the mechanical interlock means in practice
The interlock is purely mechanical — no electrical contacts, no wiring. It occupies the space between contactors on a DIN-rail or backplate assembly. When you energize the forward contactor, the interlock bar physically prevents the reverse contactor from pulling in, even if its coil is energized. That satisfies the safety requirement for a positive-break separation between power paths in reversing motor starters.