Small cap for quickly greasing LM8UU (8mm) or LM10UU (10mm) linear bearings.  The cap is designed to attach to a 3oz tube of Super Lube Synthetic Grease* and fits inside the linear bearing.  Line up the holes in the cap with the tracks in the linear bearings, slide it in, and squeeze the tube.  For double length bearings, you can do it from both sides.
If you need a cap, here's one from markwal
Printer: Eustathios
Filaments: (full list can be found on thrinter.com)
-- 3D Supply Source Transparent Purple
-- 3D Supply Source Transparent Orange
*affiliate link