Those two are the shirr plate and underbraider. To use, you remove the front slide on a long bobbin machine and put one in place. They are machine specific. I have lots of them made by Griest - they are part of the attachment set.
Here are a few just to give you an idea
This is a White long bobbin set
And a much newer White Rotary set.
And a National long bobbin set
And National Rotary set
