It is already pressed in (so pics and measurements are difficult)but the cut is 12mm wide and it is curved.
It looks like a concave shell mill was used to notch it.
Looking at the picture, think of moving right to left across the extractor cut, as you start moving it bulges out towards you then recedes back.
Looking closer I see that is is curved to make sure the case head is supported. If it was cut with a square end mill the curve of the chamber would cause a "v" across the extractor notch and thus an unsupported case.