Hello kevin_v,
These old revenue stamps could be often found in a mixed lot of old stamps (pre WWII). I reckon they were exported from Japan in bulk to be used for making stamp packets - yes, they do look like stamps but far less costly for the stamp dealers to gather.
As like the revenue stamps worldwide, they may have specific usage purposes. This particular ones were used to be attached to bills and receipts issued by inns, restaurants etc. to prove that the food and entertainment tax has been correctly paid (or collected by the inns, restaurants etc.). Those bills and receipts have been kept for fiscal purposes for a certain period, and then presumably sold to waste paper dealer and/or antique dealer, where the revenue stamps were peeled off and then sold to stamp dealers. The unused ones might be coming from old stock, which also were sold to stamp dealers.
I am especially interested and specialized in the early documentary revenue stamps of Japan (1873-1892 series), used to pay the documentary tax on sales certificate and contracts. The revenue stamp itself is beautiful, and the documents - beautifully hand written - are so so attractive. Here's an example from my exhibit leaf (sorry its in Japanese) to show its beauty - hope you like it !

- Hironobu