It looks rather like a Lyman 1A:

Images from (of course) the 1939 Stoeger catalog:

Tomato, tomatoe?

I would call all of these 'aperature sights'. The catalog descriptions refer to the smaller aperature that flips down on some models as a "turn down peep".

Here is someone else's attempt to tell us what a 'diopter sight' is: Ask Foghorn

Diopter and globe sight picture (diopter rear, globe front sights).

An optician's point of view regarding another applied use of the word 'diopter' (from Lindsey Optics):

Diopter is a unit of measurement, much like other units of measurement such as millimeters, meters, inches, etc. It's not a physical thing that you can pick up and hold. It's a unit of measurement.

We don't call lenses "Millimeters", We call them lenses, with a notation of focal length measured in millimeters. So for example we could describe a specific lens as a "50mm f2 lens" we wouldn't call it a "Millimeter".

Likewise, we should refer to close-up lenses just that way, as close-up lenses. So one description would be: "138mm Round +1 Diopter Close-Up Lens". That describes the size of the lens (138mm diameter), the power of the lens (+1 diopter) and the type of lens (Close-Up Accessory Lens).

Now all of that said, it seems that "Diopter" has become the standard name for these things and despite some annoyance, we've used it too, because it's standard. Stupid, but standard . . .