Rope Splicing Fid for 1/4 inch Rope, Metal, Marked "5/16 (8 mm)" 
Rope splicing fids are used to help run the end of a rope back through the rope when splicing or attaching tow rings to the end of a rope. 
This fid is marked "5/16 (8 mm)", but the hole seems to work well with a 1/4 inch rope.  A 5/16 inch rope does not fit into the end of the fid.

These metal fids have much thinner walls than the plastic fids and therefore have a smaller overall diameter - making them easier to push through a rope.

Dimensions
Length: 6.7 inches (17 cm)
Inner Diameter: 0.20 inches (5.1 mm)
Outer Diameter: 0.281 inches (7.1 mm)