I’m working on re-building an old plane of a style I like that was engineered in a way I dislike. Which is looking like replacing about half the parts. This part is ¼” steel with a brass overlay that is attached with bronze rivets. The brass/cocobolo screw is a part I got from a Bridge City blowout, I made the stainless steel threaded part.
The rivets are 3/32″ silicon bronze welding rod (because I like the color and the contrast with the brass). I mushroomed the head by clamping it in a mill vise and peining it. A wire wheel smooths it out. The holes in overlay are counter sunk so the rivet head can nestle. Then I mash the backside, great fun.
It has a curve, which looks nice and will keep the screw close to square (to the blade) when tightened (I really dislike the looks of tilted screws). There may be other reasons as Stanley scrub caps have the same curve.
The groove is very important – it will capture the cross rod. It doesn’t take much. If it were not there, the part wants to slide up (loosen) as the screw is tightened.