Notice that the sight in these photos does not have the release button along the right hand side as found on the MS 'folding peep sight' and on the Mauser sight shown in the '39 Stoeger.

I'm starting to wonder how many varieties of these existed and who made them?

Here are a couple of images (swiped from the 'net) of a somewhat different folding peep on a 'Mauser Guild Rifle':

This one's on a .360 Express double rifle:

Here's a different one, on a Merkel double rifle: