A customer recently asked me to remove a portrait painting from its frame so she could ship it. I have never seen anything stretched this way. The liner frame was used as the stretcher, and the painting was affixed/stretched to the inner rabbet of the liner with crude strips of nailed-in wood, and secured with a bunch of tacks all around on the back (which then had paper glued over them). It looks like they cut the corners of the painting's canvas to accommodate this odd technique. Needless to say, I left it in the liner, and just removed that from the big, heavy frame. This was framed at a local photography studio in 1966.
Rick
Rick