The G plug (E70100005) and a XP plug (E70200010) look very similar but they are not interchangeable.  Physically they will both seat in each cavity but the seals are designed differently to create proper sealing in the direction that the high pressure fluid interacts with the seals.

The XP Plug is designed specifically for Cavity 1 where a relief valve is traditionally used.  It is used to seal that cavity when a relief valve is not in the system where pressurized oil pushes against the end of the cartridge.

The G Plug is designed to be used in cavities where pressurized oil passes across the middle of the cartridge.

You can identify the XP plug vs. the G plug by the orientation of the seals on the plug.  See the images below for details noting the position of the seals on the end of the cartridge:

XP Plug – E70100010 Side Profile

G Plug – E70100005 Side Profile