That is correct!
During the Civil War, there were a severe shortage of metals available to mint coins, and therefore the rounds were created. The stamp adding proved that "somewhere someday" paid that amount to buy the stamp, so the stamp carried the monetary value with it, so to say. It is a really interesting monetary curiosity that even sometimes is commented on Economy College classes.
The other interesting part is that there was attempt of having a "Republican money" and a "National money" (the two bands in the war) and with this type of "coins" you were able to exchange into both sides without showing the source of your funds or getting yourself in public.
Source: Me , a Spanish numismatist ;-P