I have a Gew.88 based sporting rifle, possibly made by Haenel, and retailed by Eduard Kettner, whom is still in business today that has iron sights and a tang mounted diopter. The diopter pin hole sized viewing hole is elevation adjustable. This aperture is in a sliding piece, apparently held by friction, that can be moved up and down in a dovetailed notch. This dovetail and sliding aperture piece are on the back side of the diopter so they are not visible unless the diopter assembly is up.

When this tang mounted diopter is up and the bolt cycled the bolt pushes the diopter assembly back into place in the stock. It does not pop back up on it's own.

I currently have no photos of this rifle to show you.

Vlad