The 71922-249LF: The 4-wall shrouded housing with latch lock/eject hooks and a keying slot guides the mating half and prevents mis-insertion.
Position count — loaded vs unloaded
The 71922-249LF carries 50 positions in the housing but loads only 49 contacts. Position 50 is left unpopulated at the factory, providing a keying feature that prevents mating with a fully populated receptacle unless the receptacle also skips that position.