Board Stack Fit and Contact Finish
The 71991-423LF: The mated stacking height is 11.04mm, so the board-to-board gap is fixed once the header is pressed in. If your enclosure has a tight z-height budget, verify that the header's post length and the receptacle's insulation height (0.275") together clear the lid or adjacent board. The thick tin layer resists fretting corrosion better than a flash coat.
Mounting and Orientation Flexibility
The receptacle accepts mating from either the bottom or the top entry, which means the same part can serve as a pass-through on a mid-plane board or as a right-angle mezzanine connector depending on how the header is oriented. No need to stock two different part numbers for the two orientations.
The base product number is 71991, and the -423LF suffix indicates the 46-position, tin-plated variant in a tube package. No direct pin-compatible alternative is listed from the manufacturer, so stick with this order code for a drop-in replacement.