Lens Vade-Mecum types: what is this thing?

[walter23]

Other view:


Looks like any other old brass rack & pinion lens except for that weird big diaphragm on the front. I wonder if it's missing something or if this is the whole thing? Maybe it's a landscape lens?

Saw it on ebay. It's now sold, but I'm wondering if it was anything weird or interesting.

[Steven Tribe]

I think this looks like a universal iris (not a mounting iris) which, as you say, has been used in conjunction with a iris-less barrel lens.
The tailboard (mono/stereo?) still has some hint of its former glory!