I acquired an example of this Remington UMC marked pinfire label. Mine is just the label attached to the top of the box it was stuck to and cut into that square with the corners cut at an angle.
It is photographed sitting on top of the box pictured below.
There is also an example (in black and white) in the Zwecker guide which years ago I had colored to imagine what I assumed it probably looked like. The Zwecker one appears to be an over-label on top of an existing label. However mine does not appear to have a label undernear it. I was able to pull up the corner a bit and it seems to be just the cardboard under it.
Also, just for reference, here is an even earlier variation.