Here is the evidence that my little sister's bike formerly had a chainguard - there are two (well three) mounting points below:
I found her an inexpensive but pretty chainguard at Monotomy Vintage Bicycle, and now I hope for help in deciding how to mount it.
The top point is easy, there is a hole drilled in the chainguard, we just have to find a bolt that won't interfere with the chainring.
The rear connection is more complicated. That mounting bit is meant to have another piece and attach to the chainstay.
I wonder if it would be better to get a different piece of metal, attach and rotate that one quarter turn clockwise, drill another hole further back through both the chainstay and the metal piece to stabilize it with a second connection, and then mount it to the frame with the frame's rear attachment point?
Or perhaps there's yet another option I'm not considering?