Most often, there is only one screw on the right. On the left, there are usually three or four screws. That's because the vacuum port is usually on the left, and there is a gasket between the mounting block and the stack. So those screws actually serve a dual purpose. Mounting and sealing.
Some units only have mounting brackets on each end with one screw each.
And there are a couple of units that have a mounting bracket near the center of the stack, which connects to the keybed. Once you feel that everything's been disconnected, gently try to move one end towards you about one inch. If you encounted any resistence, stop and look. Find out what's not letting go. Then do the same on the other side.