The "wedge shape" is simply the mitre effect of the vertical hole meeting the horizontal one, shown in green.
The CSK area shown in red is what the valve seats against and when that is open it lets the air out from the cylinder via the cross drilling then passed the valve seat and finally down the exhaust hole as shown in blue.
Also are the steam inlets holes bored into both cylinders as you mentioned in the thread or is it just in the intake side.?
Not sure which hole you are refering to and there is only one cylinder, can you give a bit more detail.