Unusual vintage panty slip?

[BChapman]

This is a recent estate sale find at an old home in Los Angeles. The fabric is silk. I've searched for and found similar half slips with a strip of fabric that kind of makes a "crotch" (step ins) but cannot find anything else like this. There are two flaps on the inside and a single button to make a full panty inside. Any ideas as to what it's actually called and age. Any help is appreciated.

 

[poppysvintageclothing]

That is unusual. I've never seen one quite like this. Wish I could help with a name for it, perhaps someone else here has run across one like this.
Nice find!