On the two separate 4-108 engines that I used to own, the flange on the water pump
itself was the same size as the plate that you are calling the adapter plate.
pump was fastened directly to the timing gear
cover with the 4 studs which show in your second photo, the photo without the pump attached.
Because the four studs are attached to a free floating ring on the inside of the timing cover, and because the timing cover itself was just formed sheet steel
, there always was a problem keeping this joint oil
tight with just the standard gasket
. Consequently the pump flange always had a healthy layer of gasket
goo of one description or other sealing it to the timing cover.
From your photo there also appears to be a layer of sealant
on your adapter plate, and I think this is what is holding the assembly to the timing cover. Make sure all four of the adapter attachment studs/ bolts are removed and the adapter should come off with some persuasion.