Yes, exactly! This is what I have been trying to tell you. If the other end of the drive cable was attached to the motor end it would make the piston move. This is not happening!! This nut will just spin in either direction, left or right just by holding the cutting head!! Actually, you don't even need to hold the cutting head!!!
now, if you were to lock the black plastic bumphead in vise, and turn the nut to the left it should unscrew right off. If the shaft is turning in the tube,
then the bump head is stripped out. Because the two mating sides of that look like this:
View attachment 21656
View attachment 21657