I would state that it is a fake overprint with a possibly fake cancel, doubly or triply struck.
Here is one pictured on
ebay now from seller wild_rose_stamps . Note the width of the overprint. Well within the borders of the design of the stamp.

The cancel / postmark seems contrived a bit. The '0' in the center does not seem to belong to anything. Perhaps I am jumping to conclusions a bit but two or three strikes of a postal dating hammer does seem a bit odd for the time.
In my older 2005 Unitrade cataalogue the Earliest Recorded Date (ERD) (of usage) is July 31, 1899 and the cancel on the stamp reads 1898 I think, a year earlier.
The cancel is very hard to read. It should read a two-letter month and a numeral day date, with a two-numeral year date beneath.
Sometimes the date looks like '00 20 / ??' or '20 FE / ??' with a '98' somewhere floating around. Not good. Maybe my eyes though.
This is really neat to have in your possesion as a comparison to real ones. If sold it should be mark fake on the back and advertised as such.