the on/off to the left is a old one. They aren't too consistent and you'll start experiencing shoot down at say 6-8 bps compared the the right one at 13 to 15 balls per second.
As for the milling on the body of the valve, better performance wasn't the reason. I've seen this a few times and was another option for those valves that didn't have the body and reg holes lined up. As you can see how the milling stops 180* from the body hole which would line up on those valve who's regs were drilled wrong.