This 5mm extension ring is often used for professional field flatteners such as the SharpStar 2.5" field flattener.
The extension ring has an M68 x 1 male thread on one side and an M68 x 1 female thread on the other side. The total thickness is 9mm which includes the 4mm male thread.
|Connection (side a)
||M68 x 1 male
|Connection (side b)||M68 x 1 female|
|Optical thickness||5 mm (not including 4 mm male thread)|
|Clear aperture||63 mm|