I've been conversing with the guy in the thread from MTU "crash" and getting the info from him... Those holes around the groove that I have circled are the input holes. THe fluid comes in those and then is directed to the other ports through slots as the steering is turned in either direction. As it sits still there are no slots for the fluid to move through, therefore causing the fluid to hit a dam with constant pressure of more fluid being pushed at it. I'm sure you understand that part, but posting it for those like me who didn't understand it at first. I will get a pic of the spool and the slots I'm referring to today and post it up. It's up at the machine shop waiting on them to get freed up to drill it for me.
BTW, crash is a super nice guy and was very eager to help with this, so I told him to please copy and paste the pics to their thread seeing as it was the only one that came up when searching for doing this. Maybe even link this thread in theirs...