I was looking at a picture of a mid 80s touring bike by Centurion that has a little do-dad on the down tube. It appears to be clip that is designed to fold down and keep the front wheel from rotating and in-line with the bike frame. I've never seen this before and wonder what larger purpose it serves.

You got it Pontiac.
thats about it, no larger purpose.

You Got It Pontiac 1985 - YouTube

Basically, it's a parking brake. There are some flip-down metal brackets that are designed to sweep thorns out of a tire on some mountain bikes. neat tool for riders in AZ/NM/UT

Rhode gear flick-stand.
On a few long distance rides with roadies I have taken on my 85 team Fuji, several people have been interested when I can flick it down and lean the bike against anything without the wheel flopping. Its interesting because some of them want one, partly I think because they have seen it in action and its actually fairly useful, partly because they cant have one, as they wont fit their larger diameter or oddly shaped frame tubes, but mostly because they are not made anymore.

@ mdlthey I would love to see a picture of a flip down thorn sweeper for a mountain bike. I have several from the 70s and 80s that were made for road bike tires, but cant see them working for knobbies.

Thanks, everyone, for your input and explanations.

Yup, Rhode Gear Flick Stand. Long out of production, but a very useful gizmo, especially for touring bikes (can't use with fenders, though). I have one on my 26" touring bike. See below.

I like those water bottles . . . . what are they, quart size?

Liter ... French ..

Zefal Magnum, one liter. Nice in that they fit in standard bottle cages, assuming you have enough open frame space. I have a bunch of them, we use them a lot on our tandem/triplet rides. Only complaint is that they have a sort of "washer" in the lid, and if you lose that then they tend to leak/drip.

Magnum - Zéfal