Rounded eyelet (pattern)
This looks like a fairly complex pattern made up of larger, filled octagons joined together by smaller straight squares. In actual fact the pattern is easy to follow by completing a number of offset eyelets and then outlining each eyelet with a octagonal frame.
Method
Start by completing a single diagonal stitch across one thread and down over three threads.
Add another seven diagonal stitches of the same length all going down into the same hole to complete an eyelet.
The next eyelet should be offset from the last, but touch at the corners.
Fill the area with a pattern of offset touching eyelets.
Each eyelet should then be framed with an octagonal outline.
Continue to outline the eyelet pattern with the octagons.
Embroidery Techniques
References
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Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.151