Harry Bloomfield wrote:

A couple of years ago, I spotted the wedge just laid in the frame, as if
it had fallen out of somewhere or been displaced, so found it would fit
the frame bottom and I left it there.

Could it be a bridge packer that's fallen out? No doubt each extrusion
manufacturer has their own variation, but you should get the idea ...

https://google.com/search?tbm=isch&q=bridge+packer