This is an adapter for Leica M mount lenses.
...for Sony FE/E mount cameras.
Its special feature is that it can change its length, acting as an extension ring. There is a kind of focusing ring on the adapter for this purpose.
I don't have a Leica M mount lens, but this mount is good because it has the shortest flange focal distance (27.8mm) among consumer film cameras for the 36x24 (mm) frame. This means that you can use other film lenses with an additional adapter, including, for example, the M39x1/28.8 (rangefinder) or SLR mounts.
When folded, the adapter is 9.8mm long. This corresponds to the difference between the flange focal distances of the Leica M and the Sony FE/E: 27.8 - 18 = 9.8 (mm). As I mentioned, I don't have any Leica M lenses to test, but with additional adapters infinity is achieved, there is even focusing beyond infinity (depends on the adapter).
The maximum extension length is 14.7mm, giving an analog of a 4.9mm (14.7 - 9.8) extension tube.
Not that much, of course, but that's the dimensional limit. Even this extension reduces the minimum focusing distance.
Installed lens does not rotate when adapter length is changed by rotating the ring.
Another aspect is that helicoid threads is open to the camera. You have to be very careful with the grease.
The main disadvantage of the adapter is that its construction is rather wobbly, there is a lot of play. My copy is used (I bought one to save money), but I think it's the same with new copies.
But this is one of the cheapest models of the Leica M-Sony FE/E close focus (helicoid) adapter. It is sufficient for my experiments.
