Ah, Mississippi, home of the "Delta Blues" & Robert Johnson (May 8, 1911-August 16, 1938), an extremely influential musician in the acoustic blues genre. There has already been a thread here about him - http://goscf.com/t/27314
but this is about the guitar on the stamp issued in 1994, Scott 2857.
There are only two confirmed photos of Robert (and a third which is the subject of much discussion). The one on the stamp is based on a photo booth picture.
From this, guitar aficionados have deduced that the guitar in question is a Kalamazoo Kg-14, a budget brand made by Gibson. Zeke Schein, who works for Matt Umanov Guitars in New York City, says the identifying features of the KG-14 are "14 frets to the body, five dot markers, a single layer of binding inside the sound hole, and a black ebony nut."
In case you're REALLY interested, here's what one sounds like.