The pictures were on PukeBucket.
If you look closely at your hole it appears the edge of the through hole is not smooth. The problem with this is the ball sits on that edge, not the face of the seat, so it does not seal.
With a standard 118 degree point drill the ball sits on the edge of the hole. If a sharper taper is used the ball will sit on the face of the seat.
From the pump side you can see how close the bottom of the seat is to the cross hole.
The red arrow points to the bottom of the seat and the blue line is the intersection of the two holes. There is about 0.015" from the bottom of the seat to the cross hole.
Cut the seat too deep and it will intersect the cross hole and then you have to insert a new seat (disassemble the cases, machine the bore, press in a plug and re-machine).
If you want to move the contact point away from the hole you can use a smaller ball, 1/4", welded to a rod and lap the hole with grinding paste. That will make a new edge away from the rough through hole.
I made this tool to re-angle the seat and control the depth.
The collar is adjusted with a feeler gauge so it bottoms on the bolt head at the final depth.
I believe I sent one of these to Clive Blake, the TR3OC Spares Liaison in the UK.