I initially thought this might be an improper use of a revenue stamp as postage, but upon examining the postcard and reading the notation, it is not, but still a neat history piece, so I bought it anyway.

It is a postcard sent from San Francisco, California, to Basel, Switzerland, on September 2, 1898. Per images of other international postcards from 1898, the proper 5 cents postage was paid outside the revenue stamp (Scott # R155, large provisional I.R. overprint).

The handwritten message on the back reads:

Dear Sir The stamp on the margin marked “I.R.” is a postage stamp turned into an Internal Revenue Stamp of the War issue, provisionally to supply the demand before the regular War issue could be got out by the Government.

Just a neat history piece of the era with a revenue stamp tie-in.