Old Wall Mount, can't tell how to unlock

If you are lucky the bolts that attach the horizontal bar A to the wall are accessible with a socket wrench. If you can support the TV and the attached mount ( hopefully using two strong friends) you might be able to unmount the whole set up from the wall. A long shot, but you may get lucky.

If you are even luckier, the device that stops you from dismounting the display may not be screwed in too tightly and, providing nothing is in the way, you ( and your friends) might be able to slide the whole thing off of the horizontal wall bars. If you push up on the base of the display, the hooks on the top may go up a bit from the horizontal bar. That may give you an idea of whether it's loose enough to be slid off.

To my shame I once installed a display in a recessed frame that required unscrewing a horizontal screw on the bottom of the vertical bar resting on the bottom horizontal rail. I deliberately left these screws a little bit loose so that whoever tried to remove the display could slide it right and left just enough to get a long screwdriver in.