The spacer goes between the gearbox and a hop up
GR25 (g&g sr25 only)
GR16 (m4 standard unit)
HopUp spacer for gr25:
They are all pretty much 3-D printed. They are not too difficult to find. They seem to be very common for the community that is running HPA engines (p* jack, Wolverine inferno/reaper, etc)