Hydronit male-male 1/4" BSP nipple.
Male-male nipple with 1/4" BSP thread at both ends (E70060001), from the Hydronit modular mini power pack system. In the Hydronit catalogue it appears as the part for option P of the VUC20 check valve: it screws into the pressure port to connect a gauge and read the static pressure.
It also works as a union between two 1/4" BSP female threads. Check the thread of both parts before ordering.
Source: Hydronit AC & DC Hydraulic Power Packs Compact technical catalogue, version PPC2010-05/EN.
Are you an OEM? Reseller? Workshop?