A surgical stapler assembly, comprising: a disposable end effector comprising a distal attachment portion (5504 - Fig. 38), wherein said distal attachment portion comprises: a key; and a lock notch; a surgical instrument, comprising: a proximal attachment portion (5522 - Fig. 38) structured to receive said distal attachment portion, wherein said distal attachment portion is moveable along a longitudinal axis relative to said proximal attachment portion between a non-inserted position and an inserted position; a firing shaft comprising a firing shaft rotator (5544 - Fig. 40), wherein said key is structured to engage said firing shaft rotator to rotate said firing shaft rotator out of alignment with a firing shaft slot (5518 - Fig. 41) and to rotate said firing shaft rotator back into alignment with the firing shaft slot when the distal end of said key moves past the firing shaft rotator as said distal attachment portion moves between said non-inserted position and said inserted position; and a rotatable collar that is rotatable relative to said longitudinal axis between a locked position and an unlocked position, wherein said collar is biased toward said locked position, and wherein said collar comprises: a groove engaged with said firing shaft rotator, wherein said firing shaft rotator is structured to rotate said collar to said unlocked position when said key is engaged with said firing shaft rotator; and a detent (5582 - Fig. 46) structured to move into said lock notch when said collar rotates from said unlocked position to said locked position when the distal end of said key moves past the firing shaft rotator and when said distal attachment portion is in said inserted position. a disposable end effector comprising a distal attachment portion (5504 - Fig. 38), wherein said distal attachment portion comprises: a key; and a lock notch; a key; and a lock notch; a surgical instrument, comprising: a proximal attachment portion (5522 - Fig. 38) structured to receive said distal attachment portion, wherein said distal attachment portion is moveable along a longitudinal axis relative to said proximal attachment portion between a non-inserted position and an inserted position; a firing shaft comprising a firing shaft rotator (5544 - Fig. 40), wherein said key is structured to engage said firing shaft rotator to rotate said firing shaft rotator out of alignment with a firing shaft slot (5518 - Fig. 41) and to rotate said firing shaft rotator back into alignment with the firing shaft slot when the distal end of said key moves past the firing shaft rotator as said distal attachment portion moves between said non-inserted position and said inserted position; and a rotatable collar that is rotatable relative to said longitudinal axis between a locked position and an unlocked position, wherein said collar is biased toward said locked position, and wherein said collar comprises: a groove engaged with said firing shaft rotator, wherein said firing shaft rotator is structured to rotate said collar to said unlocked position when said key is engaged with said firing shaft rotator; and a detent (5582 - Fig. 46) structured to move into said lock notch when said collar rotates from said unlocked position to said locked position when the distal end of said key moves past the firing shaft rotator and when said distal attachment portion is in said inserted position. a proximal attachment portion (5522 - Fig. 38) structured to receive said distal attachment portion, wherein said distal attachment portion is moveable along a longitudinal axis relative to said proximal attachment portion between a non-inserted position and an inserted position; a firing shaft comprising a firing shaft rotator (5544 - Fig. 40), wherein said key is structured to engage said firing shaft rotator to rotate said firing shaft rotator out of alignment with a firing shaft slot (5518 - Fig. 41) and to rotate said firing shaft rotator back into alignment with the firing shaft slot when the distal end of said key moves past the firing shaft rotator as said distal attachment portion moves between said non-inserted position and said inserted position; and a rotatable collar that is rotatable relative to said longitudinal axis between a locked position and an unlocked position, wherein said collar is biased toward said locked position, and wherein said collar comprises: a groove engaged with said firing shaft rotator, wherein said firing shaft rotator is structured to rotate said collar to said unlocked position when said key is engaged with said firing shaft rotator; and a detent (5582 - Fig. 46) structured to move into said lock notch when said collar rotates from said unlocked position to said locked position when the distal end of said key moves past the firing shaft rotator and when said distal attachment portion is in said inserted position. a groove engaged with said firing shaft rotator, wherein said firing shaft rotator is structured to rotate said collar to said unlocked position when said key is engaged with said firing shaft rotator; and a detent (5582 - Fig. 46) structured to move into said lock notch when said collar rotates from said unlocked position to said locked position when the distal end of said key moves past the firing shaft rotator and when said distal attachment portion is in said inserted position.