Admittedly, this stamp is likely not a big deal to anyone but the most ardent of plate variety collectors, but when I saw it at an MSDA show last year I just had to buy it.
It appears to be a garden variety example of Scott #R115a, the invert of the 2nd issue 50-cent documentary. Centering is kinda "meh" although it's nice in that it's not punch canceled or punch-and-filled like so many frequently are.
I initially glossed over it, as I already have several examples, including one on document, but figured I would do my due diligence in the hopes that it might be what I was looking for... and it was!
It happens to be one of the 3 plate positions that exhibit the Scott-listed double transfer in FIFTY at the top! I have an example of the DT as a regular stamp, the sewing machine perf, and now the invert. The only remaining target would be the imperf, but those are incredibly rare even as the non-DT stamp.

