I just checked my *istDS with a screwmount lens and K-mount adapter. As far as I can see there is nothing in the 'throat' of the *istDS that any pins could get caught on with the possible exception of the mirror return mechanism on the left hand side (facing the camera).
It appears possible (just) that changing the aperature on the lens could cause the 'tab' (which moves in a slot) to catch on the mirror return Are you able to manually change the aperature on the lens ? If so see if the lens can then rotate.... Good luck. Fred.
