A flat head is a disc with a flanged side according a radius.  Flat heads are used in the agri-food industry for turning or distribution plates. The inner height of the head is given by the addition of the knuckle radius and the right side of the head.
As spherical caps, flat heads do not follow any standard.

The front endplate with a large hole for the furnace and small holes for the stays and the fire-tubes. Both endplates are "dished" in the traditional fashion with curved edges so that they can be welded directly onto the ends of the main boiler shell. Then will be less possible stress on and flexing of these welds than flat end plates.