The Borg M57 Helical Focuser DX is available in two lengths (photo shows the longer DX LII model).
The longer DX LII is 'standard issue' for the Borg 60ED and has the longest focus travel of all Borg Helical focusers, so is popular for terrestrial photography.
Both models have two locking screws (useful for imaging applications) and large 51mm internal diameters.
Helical focusers work by turning a ring to focus as we do when using a conventional camera (at least in manual focus mode). A true helical focuser works like a camera - when you turn the focusing ring, neither the lens nor the camera (or eyepiece and diagonal) rotate. The only motion is in and out. Some manufacturers implement a "pseudo-helical" focuser which results in a rotating camera or eyepiece, but Borg focusers are true helical focusers.
The advantage of helical focusers is the fine adjustment possible, in part due to the fine pitch of the focusing ring's thread, and in part due to the large diameter of the ring. In the case of the Borg helical focusers, virtually backlash-free movement and graduated markings on the focusers allow repeatable return to the focus point simply by noting the focus position reading.
Specification M57 DX 7758
Index : 100 microns
Length : 26 to 43 mm
Weight : 80 g
Outer Diameter : 69 mm
Inner Diameter : 51 mm