That cut probably was made with a single, shaped milling head.

If I was to duplicate, I would use a router.

Cut the slot to match with a straight bit of the right width. Can also be cut on a table saw with care. Then I would put the radius on the sides of the slot with a rounding over bit (the term I know). It appears that the two edges have different radius corners, but that just may be the photo.

Easier said than done.