Let's play "What the heck is this?"

It is an odd object. What is the lensboard made out of and how big is it? I wonder if it goes with the very first generation of Speed Graphics, which had a smaller lensboard. I've never actually seen one of them.

I think it's a taking lens. It might be for a simpler camera than I was expecting, because it's a simple lens, but only a taking lens would have Waterhouse stops. DOF preview wasn't really a common thing until automatic aperture lenses for SLRs came into being. Prior to that, judging DOF meant looking at a DOF scale, or looking at a ground glass through the taking lens at working aperture.

To be a viewing lens useful for focusing, it would have to be matched in focal length and mounting to the taking lens fairly accurately (better than a fraction of a millimeter), as is done for TLRs.

Yes, an odd object. For any argument on its use there is a counter argument. Likely thus it was not successful... and we did not know about it.