definitely looks like something intended to hold an insert into a recess at the frame / perimeter. the rounded end of the thumbscrew would indicate that it was to bear against glass. Maybe for a mirror. The S-shape os to accommodate different thickness and provide additional tolerance.

That's all I got.

That type of bracket is used in cabinets to hold glass in place from the back side. For example, if you have a china cabinet with many glassed openings in the doors, it's a lot of work to do a recess in the back of the door for each opening and then to cut the individual panes. Instead, a single large piece of glass is cut to cover all the openings and these brackets are screwed in place around the perimeter on the back side of the door to hold the glass in place, butted up to the frame of the door. This type of construction has a definite benefit when it's time to clean the glass. You simply unscrew a couple of brackets; lift out one large piece of glass; clean both sides; and then screw the whole thing back in place. This is much faster and easier than trying to deal with individual small panes that may or may not be removable.

Does that make sense?

Kudzu9 - thank you! I knew because of the rubber tips it was probably for glass. What you described explains the s-shape. It still seems that the rubber tipped anchoring screws would stick out pretty far with this design. I also thought that they were sent (accidentally) with our kitchen cabinets (we didn't have glass doors) which also made me think "glass fronted cabinets". Either way, I'm giving them all to a local place like ReStore

drmeow-
Yes, they can stick out a bit. Looking at it again, I realized that there is nothing to prevent them from being used with recessed glass also, and then they screw in a bit further (to a little more than the position shown in your photo). It's perhaps not the most elegant bracket, but it is traditional and its shape and the length of the screw does allow it to handle a wide range of glass thicknesses and configurations.

Typical on older cabinet doors that had glass inserts. I'm certain you can still buy them. They have the advantage that the glass can easily be removed periodically for a good clean, which is important if there are faux muntin grilles in use, because you really cannot clean around them adequately.
Casey

They're available from Rockler. I'm thinking of using them for my glass front cabs.

Here is a link that might be useful: Glass retainer

Rockler also has another style if you want a lower, cleaner profile. They don't have a screw adjustment, but you get two lengths of rubber tips and you can also flip the clip: the specs say it can handle anything from flush to 1/2" setback....

Here is a link that might be useful: Clips

Thanks kudzu9, I saw that one too. Since I don't plan on removing the glass, it might be the best one for me.