Ferrite pot core calculations

Fair-Rite POT Core configuration provides a high degree of self-shielding and facilitates gaping to enhance utility for a variety of magnetic designs

your absolutely bang on, more than likely early 68, the ferrite core looks like it has the MullardFerrox cube FX designated 4 slot inductor

40 hot slot free cores 212 PQ cores 281 PM cores 300 EP, EPX, EPO cores 315 P for magnetically soft ferrite materials, coincides with the initial magnetization curve

4d slot your absolutely bang on, more than likely early 68, the ferrite core looks like it has the MullardFerrox cube FX designated 4 slot inductor