Copper coil bobbin 15 mm long (ish)

A 2-part bobbin. 15 mm long (inside) and 21 mm long (outside). Diameter about 16 mm.

Designed to take a 5 mm diameter ferrite rod. It turns out the transformer I wound on this was nowhere near as good as I wanted (probably due to the incomplete magnetic path) but it may be useful to someone anyway!