The Hirose DF40HC(4.0)-80DS-0.4V(51) is an 80-position receptacle from the DF40 series, designed for board-to-board mezzanine stacking. It uses a 0.40mm pitch, dual-row layout with center strip contacts, and mates to a matching DF40 header to achieve a 4mm mated stack height.
Stacking height and board spacing
This is the dimension you design the enclosure or module gap around. The height above board for the receptacle alone is 3.90mm, so the header side contributes the remaining 0.10mm to reach the full 4mm stack.
It protects against corrosion during storage and initial mating, but it is not built for hundreds of cycles. For a connector that gets mated once during final assembly and maybe a handful of times for rework or field service, this is fine. If you need frequent connect-disconnect in a test or calibration fixture, look for a thicker gold option in the DF40 series.