Dovetail type of ( 2" to 1.25") convertor / adaptor (which also able to tread standard T ring) and also to take 1.25" eyepieces or CCD with large locking thumb screw.
This can be connected to 2" visual back or 2" APO's focusing tube.
Due to circular dove tail connection with APO or visual-back, eyepiece, ccd or camera will not fall out suddenly even thumb screw is loose.