It's difficult to tell from the picture on the page, but the hole in the peep sight itself is adjustable to increase or decrease the aperture.
I have one of those on a Win 94 and that is not how it works. The aperture is not adjustable, it is replaceable and available in multiple sizes. For some reason they don't list them on their web page, but they are listed in their catalog on page 14.
They are also available from other sources, there is a list here: http://www.gunaccessories.com/Willia.../Apertures.asp
Probably not what you are looking for, but I have something like this on my CMP rifle, fits a standard A2 rear sight:
On edit, adjustable iris sights do exist, but they are large/bulky and expensive.