There's two of these on the back of my all-in-one Acer VZ4640G-I3610Z. The rectangular part can shift to the other side of the square. The lock does not have a letter K on it. There's a Kensington lock elsewhere on the computer.
Most likely these are the catches for removing the back panel of the computer in order to gain access to the inside. The lock symbol signifies that you can put suitably-sized screws in the holes to "lock" the catches in the closed positions, preventing access without a screwdriver (this is useful when such computers are used in schools, for example).