Micro 4/3 lens adapters:
M42 focusing helicoid  

Focusing helicoid with male Micro 4/3 bayonet and M42 female attachment. It extends from approximately 17 to 30 mm by turning the scalloped ring. The lens does not rotate when the focusing ring is operated.

Pros: The built-in Micro 4/3 bayonet allows a lesser minimum extension than focusing helicoids that require the use of a Micro 4/3 adapter. Solid construction (evidently the same construction as commonly available M42 male-to-female helicoids). The interior is painted flat black to avoid internal reflections.

Cons: Black anodyzed aluminium rear bayonet, integral with adapter body.