Kipon Icarex to Micro 4/3 adapter. Purchased on eBay.
Pros: Solid one-piece construction.
Cons: Rather expensive compared to other adapters of similar design. Black anodyzed aluminium rear bayonet, integral with adapter body. Interior is relatively shiny and may need to be painted flat black or lined with flocking to avoid flare and loss of contrast. The Icarex bayonet of this adapter seems a bit too thick to mate well with some of my original Zeiss Ikon adapters, while it works satisfactorily with others.
Zeiss Ikon marketed the Icarex SLR cameras in the mid-20th century. The first of these cameras had a peculiar male bayonet on its camera body (generally called Icarex 35). Lenses and accessories were attached by rotating a broad fastening ring. The attachment is very solid and does not exhibit any play or twist. For this reason, it is possible to use stacks of several extension tubes without introducing wobble or play. Two types of extension rings (25 and 50 mm long) are commonly available on the second-hand market. The microscope adapter shown above is slightly less common. Another interesting feature of these accessories is that they have built-in baffles to reduce the chance of internal reflections. They can be useful in situations where an extended length of tubes and a rigid connection are required.