The tang mounted diopter sight on the Mauser rifle posted by Rothammer1 (two postings above this one) is very similar to the one on my Gew.88 based sporting rifle. The primary difference is that mine is engraved with the rest of the rifle embellishments.

You can see on the one posted by Rothammer1 that the diopter aperture elevation is adjustable by sliding the strip of metal that the aperture is in up/down in the dove tailed track that it is mounted in.

I believe the diopter mounted on the Mauser rifle in the photograph is one that will be pushed down when cycling the bolt but will not return upright on it's own.

Vlad