Popular for EFHW anennas,  baluns, PA stage, EMC filters, antenna choke baluns, etc.
As used in AI6RW balun/OCF dipole.  (Google for full info).

      
31 MIX IS RELATIVELY NEW AND HAS GREAT BROADBAND CHARACTERISTICS OVER THE ENTIRE HF/VHF RANGE.

Optimum frequency range <1 to >300Mhz
1.12" o/s diameter  (28.5mm)
0.54" inside diameter  (13.8mm)
1.125" Height  (28.6mm)
Weight 68g

THIS LISTING IS FOR 1 FERRITE CORE


Full data ;  google part number  

Low price, free postage.