Here is the detail of buttonhole integrated in seam between collar/jacket and collar/facing:
Per instructions taken from 1935 DuBarry trench coat:
Stitch at neck edge matching oo and easing ‘F’ to coat; leave right end of seam open between oo in ‘A’ for buttonhole. Turn up back seam between oo; clip and crease seam open in front of o.
Join Front Facing ‘E’ to opposite edge of ‘F’ on inside as far as o at shoulder easing ‘F’ to ‘E’.
Leave right seam open between oo in ‘E’ for buttonhole. Clip and crease seams open. Fold ‘F’ in half and place ‘E’ on outside of ‘A’.
