Banjoman, the add-on doesn't increase the number of planes. It just lets you fly the earlier models, or shoot them down. So far, I've only encountered 2 EI's and no EII's - not much of a Scourge. Even when flying for the Germans, the EII type is a rare bird and appears for a short time.
"Take the cylinder out of my kidneys, The connecting rod out of my brain, my brain, From out of my arse take the camshaft, And assemble the engine again."