This 8 mm extension ring is often used for professionals filed flattener such as the SharpStar 2.5" field flattener.
This extension ring has a M68 x 1 male thread on one side and a M68 x 1 female thread on the other side. The total thickness is 12 mm and it includes the 4 mm male thread
|Connection (side a)
||M68 x 1 male
|Connection (side b)||M68 x 1 female|
|Optical thickness||8 mm (not including 4 mm male thread)|
|Clear aperture||63 mm|