Ferrite core E20/10/5, material 3C94.
Core of the E-E type is by definition a split core. The core can be inserted into a bobbin. An air gap can be applied between all mating faces (similar as shown in the photo) or the central leg can be ground down to provide gap just on itself, with the side legs touching.
See also: ferrite_core_e20-10-5_magnetica.jpg
e-e_core_e20-10-5_magnetica.jpg
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S. Zurek, Encyclopedia Magnetica, CC-BY-4.0
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