A flat-core ferrite used for making coaxial "patch-lead" chokes as detailed in GM3SEK's "In Practice" article.
Low Bands (1.8-3.8MHz) Choke: 3 cores, 5 turns, approx 125mm dia
Mid Bands (5-10MHz) Choke: 3 cores, 4 turns, approx 85mm dia
High Bands (14-30MHz) : 2 cores, 3 turns closely wound
Can also be used for generic chokes according to your needs/situation.