Early version of the Leitz waist-level 50mm reflecting finder, AUFSU. Introduced in 1929. The field of view corresponds to a 50mm lens. The later AIUFSU version was equipped with an accessory shoe.
It mounts onto the accessory shoe for use. If attached sideways at a 90-degree angle, it technically becomes a horizontal eye-level finder — whether that serves any practical purpose or not.
Practicality aside, this is one of those delightful little Leitz accessories that makes you want to mount it on a Leica A, DII, or DIII. It is also one of those oddly shaped finders that leaves you wondering every single time where exactly to hold it when sliding it into the shoe, and in which direction it should fit back into its case...
A compact and beautifully engineered accessory with the unmistakable precision feel of classic Leitz products. This early version is quite rare. Most AUFSU finders show heavy wear from long use, but this example is exceptionally clean.

There are slight paint losses and minor scuffs, but overall it remains in very clean condition with little sign of use. The glass shows some yellowing and haze.
Comes with the original case. A wonderful addition to any collection!