It is just a small tank for a grill.
The one I saw in the catalog actually screwed into the stem that attaches to the gas line to the grill. I have since found one like that in stores, and am currently using it, so I will see if it continues to work.
Once you put the gauge on, then you hook the line to the gauge.
The second gauge down on the page is the one I found at a local hardware store. Has anyone used this one? Does it last?