Originally the header bow was attached to each side of the top irons with a machine screw such as in your photo that was threaded into a T-nut inserted into a the header bow from the front. Obviously the hole in the bow was stepped with the larger diameter only deep enough so that no part of the T-nut protruded outside of the outer surface of the header. The second fastener per side was a wood screw.
It is highly unlikely that your reproduction header bow (assuming that is what you have) has the large stepped hole on each end for the T-nuts. If it does, consider yourself lucky. If not, it can be added or you could take a chance and use two wood screws per side. There is a risk that over time that the wood screws will loosen as a two- wood-screw attachment is not nearly as secure or strong as the original arrangement.
To avoid a depression in the top material where the large hole existed, wood plugs filled the holes once the nuts were installed which were then trimmed down to leave a smooth, hole-less surface. I use wood filler for that purpose.
(For the purist, the T-nut shown in the above photo is not exactly what was used originally as that was a slotted round nut with tapered sides, but unless you salvaged those from an original header bow you will likely not be able to find an equal to the original nut and the T-nut is a satisfactory replacement.)
I have an original header bow from which I could provide the exact location of the holes for the T-nuts, but I cannot access it until late next month. They are however self-defining as it is critical that the side of the brackets at the front of the header bow are flush with the ends of the header.
But first you must satisfy yourself that the relationship between the windshield frame, the windshield posts, and the header bow is what it should be so that when the front top iron brackets are on the dowel ends of the windshield posts that the header bow tips come to the outer edge of the top iron brackets. If not, depending on whether or not your windshield frame and the cones that fit into the cups in the upper inner part of the windshield posts are originals or accurate reproductions or otherwise, you may have to make some alterations.
