FS-1E series size 25/13/11 ferrite transformer cores made from N87. These standard 'E' type ferrite transformer cores are available with air gaps inserted:
Stock number 840-7443 has a 0.1mm gap
Stock number 840-7525 has a 0.2mm gap
Stock number 840-7608 has a 0.5mm gap
Stock number 840-7695 has a 1.0mm gap
Typical applications for these ferrite cores include transformers and inductors in switch mode power supplies
For bobbin see stock number 840-7250

Transformer Ferrite Kits - Cores

The A_L value in the table applies to a core set consisting of two cores without air gap Cores ETD conform to IEC 51(CO)276