A cassette holder of a fluid processing system, comprising: an actuator assembly comprising an actuator (40, 42), and a shaft including a head, a base, and a perimeter groove defined between the head and the base; and a cassette plate including a lever and defining a channel configured to receive at least a portion of the lever, an opening aligned with the actuator (40, 42), and a keyhole associated with the channel, at least partially aligned with the shaft, and including a necked down portion at least partially positioned outside of the channel and an open portion, wherein the open portion of the keyhole is configured to allow passage of the head of the shaft therethrough, the necked down portion of the keyhole is configured to receive the perimeter groove of the shaft while preventing passage of the head of the shaft therethrough, and the lever is movable, preferably by pivotal movement, between a locked position in which the lever is primarily positioned within the channel to prevent movement of the shaft from the necked down portion of the keyhole to the open portion of the keyhole and an unlocked position in which the lever is primarily positioned outside of the channel to allow movement of the shaft between the open portion of the keyhole and the necked down portion of the keyhole. an actuator assembly comprising an actuator (40, 42), and a shaft including a head, a base, and a perimeter groove defined between the head and the base; and an actuator (40, 42), and a shaft including a head, a base, and a perimeter groove defined between the head and the base; and a cassette plate including a lever and defining a channel configured to receive at least a portion of the lever, an opening aligned with the actuator (40, 42), and a keyhole associated with the channel, at least partially aligned with the shaft, and including a necked down portion at least partially positioned outside of the channel and an open portion, wherein the open portion of the keyhole is configured to allow passage of the head of the shaft therethrough, the necked down portion of the keyhole is configured to receive the perimeter groove of the shaft while preventing passage of the head of the shaft therethrough, and the lever is movable, preferably by pivotal movement, between a locked position in which the lever is primarily positioned within the channel to prevent movement of the shaft from the necked down portion of the keyhole to the open portion of the keyhole and an unlocked position in which the lever is primarily positioned outside of the channel to allow movement of the shaft between the open portion of the keyhole and the necked down portion of the keyhole. a channel configured to receive at least a portion of the lever, an opening aligned with the actuator (40, 42), and a keyhole associated with the channel, at least partially aligned with the shaft, and including a necked down portion at least partially positioned outside of the channel and an open portion, wherein the open portion of the keyhole is configured to allow passage of the head of the shaft therethrough, the necked down portion of the keyhole is configured to receive the perimeter groove of the shaft while preventing passage of the head of the shaft therethrough, and the lever is movable, preferably by pivotal movement, between a locked position in which the lever is primarily positioned within the channel to prevent movement of the shaft from the necked down portion of the keyhole to the open portion of the keyhole and an unlocked position in which the lever is primarily positioned outside of the channel to allow movement of the shaft between the open portion of the keyhole and the necked down portion of the keyhole.