Fair-Rite flat cable core for noise supression.  Ideal for wrapping your power leads through the cores to remove unwanted HF.
 
Core specification.
Product                  : Flat Ribbon Cable Cores Split     
Impedance           : 169 Ohms     
Inside Length      : 26.15 mm     
Outside Length  : 38.85 mm     
Outside Width    : 28.6 mm     
Inside Height       : 12.95 mm     
Outside Height   : 26 mm
 
Please search for GM3SEK to see these cores in use.