There are plenty of examples of large revenue stamps, but what is the smallest one?

Using the good old Penny Black as a useful comparative measure for surface area (of its design) which I measure at 19x22.5mm or 427.5sq mm (thus giving a "PB" unit).
Here are the three smallest I have, all record copyright tax stamps.
The little circular die-cut one is Australian, from J. Albert & Son, dating from 1957-62, affixed to some UK specialist 78s imported into Australia, notably Mood Music. It has a design diameter of 8mm, so an area of 50.25sq mm or .1175PB
The purple and green item dates from 1925-28, and is for AMMRE, from Germany. It was a smaller version of the usual stamp, meant for affixing to the smaller labels of little 78s - many under 8" rather than the usual 10" or 12". The design measures 11x8.5mm, or .219PB
The AMMRE LIZENZ CLAUSOPHON is from the 1920s, especially for the small records of the Clausophon label, and the text area measures 8.5x7.5mm, or .149PB.
Anyone got other really small revenue stamps?
Should we measure to the perfs, and if so how - to the bottom of the perf holes?
Adam