A surgical instrument, comprising: at least one drive system comprising an attachment member configured to generate control motions upon actuation thereof and defining an actuation axis, wherein the at least one drive system is operably supported by a frame; at least one interchangeable shaft assembly comprising a shaft attachment module, a closure tube attachment yoke, and a yoke arm protruding proximally from the closure tube attachment yoke, and a lockout assembly, the lockout assembly comprising: a proximal lockout slot segment formed in the frame for receiving at least a portion of said attachment member therein; and a distal lockout slot segment in the shaft attachment module opening into the proximal lockout slot segment when the shaft attachment module is completely attached to the frame; wherein the yoke arm comprises a downwardly open hook formed thereon, wherein the shaft attachment module is configured to be removably coupled to the at least one drive system in a direction that is substantially transverse to the actuation axis and transmit the control motions from the at least one drive system to a surgical end effector operably coupled to said interchangeable shaft assembly; wherein the yoke arm is configured to capture the attachment member in the downwardly opening hook and drive the attachment member to the bottom of the proximal lockout slot segment when the closure tube attachment yoke is moved into operable engagement with the attachment member, and wherein the at least one drive system is configured to drive the attachment member in a distal direction along the actuation axis to enable said attachment member to pass from the proximal lockout slot segment to the distal lockout slot segment to result in closure of an anvil of the surgical instrument or result in application of a corresponding actuation motion to an end effector operably couple to the interchangeable shaft assembly. at least one drive system comprising an attachment member configured to generate control motions upon actuation thereof and defining an actuation axis, wherein the at least one drive system is operably supported by a frame; at least one interchangeable shaft assembly comprising a shaft attachment module, a closure tube attachment yoke, and a yoke arm protruding proximally from the closure tube attachment yoke, and a lockout assembly, the lockout assembly comprising: a proximal lockout slot segment formed in the frame for receiving at least a portion of said attachment member therein; and a distal lockout slot segment in the shaft attachment module opening into the proximal lockout slot segment when the shaft attachment module is completely attached to the frame; a proximal lockout slot segment formed in the frame for receiving at least a portion of said attachment member therein; and a distal lockout slot segment in the shaft attachment module opening into the proximal lockout slot segment when the shaft attachment module is completely attached to the frame; wherein the yoke arm comprises a downwardly open hook formed thereon, wherein the shaft attachment module is configured to be removably coupled to the at least one drive system in a direction that is substantially transverse to the actuation axis and transmit the control motions from the at least one drive system to a surgical end effector operably coupled to said interchangeable shaft assembly; wherein the yoke arm is configured to capture the attachment member in the downwardly opening hook and drive the attachment member to the bottom of the proximal lockout slot segment when the closure tube attachment yoke is moved into operable engagement with the attachment member, and wherein the at least one drive system is configured to drive the attachment member in a distal direction along the actuation axis to enable said attachment member to pass from the proximal lockout slot segment to the distal lockout slot segment to result in closure of an anvil of the surgical instrument or result in application of a corresponding actuation motion to an end effector operably couple to the interchangeable shaft assembly.