FS-1E series size 42/21/20 ferrite transformer cores made from N87. These standard 'E' type ferrite transformer cores are available with air gaps inserted:
Stock number 840-7468 has a 0.1mm gap
Stock number 840-7541 has a 0.2mm gap
Stock number 840-7639 has a 0.5mm gap
Stock number 840-7711 has a 1.0mm gap
Typical applications for these ferrite cores include transformers and inductors in switch mode power supplies
For bobbins see stock numbers 840-7288 and 840-7282

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