An old thing dredged up from things put away long ago.

So here we have a (supposed) British surcharge on a Chinese 1/2c stamp, used for payment of a late fee charge for carriage by traveling post offices, probably meaning railway post offices. The stamp has a violet (British: purple) undated rimless RAILROAD POST OFFICE/TIENTSIN.
This piece also has a 1/2a CEF overprint (India SG C2) tied by an F.P.O. No. 20 cancel dated 2(0?) AP 01 cds.
More details on this service (April 20 to May 20, 1901 only) are in the Robson Lowe British Asia book and, more completely, here:
https://books.google.com/books?id=X...rint&f=falseThere is a quote on another board from the Gibbons catalog. Except I can't find this in the Gibbons China volume, or my old Commonwealth volumes (not after India or Hong Kong).
As noted above, there are no mint and the unused balance was destroyed when the stamp use and extra charge was ended.
So, anybody think this is not genuine? Images online that I found are too small to judge well. I note that there is a tiny serif sticking up on the upper right of the "F" of "Five" that does not show in those small images.
The CEF stamp on the piece belongs, being the initial rate without surcharge. Whether F.P.O. 20 was in Tsientsin at the time, I do not know. It would be nice to have the whole cover, but it is what it is. It does appear to be cut from a cover because of the corner rips; forgers and favor cancellers like to make their creations nicer and neater.
The green overprint was used after the black one, so based on the CEF cancel date, this piece makes sense.
I suppose I will need to send this in for certification (somebody did for theirs but used PSE(?)) but I thought I would get some opinions and show off this oddball item.