Kev......following is an image of a model Stuart steam hammer.
...... is this the part in question?....
If so this builder appears to have the flats on the rod at 90 degrees to the Drawing
If you could manufacture a round broach, then machining it back to a 1/2 round profile [+ reduced width] ....assembling the components & broaching both passes in both components would be the obvious procedure
The 77733 x 7/16" image in 'special shape' format is what I was trying to explain......
Clearly the design is to achieve an anti rotation of the hammer head....so if the equivalent flat surface are is provided by one larger flat over two shorter flats could be worth considering
Certainly a high wearing component assembly
Derek