This first shape is great shape to get familiar with in all keys. You can find this shape in any key by locating the root note on the B string (8th fret in this case). After the first few accents on this chord, we slide up to a G13th on the 12th fret of the G, B & E strings, and then move chromatically down to a G9th on the 10th fret of the G,B & E strings.
It's worth transposing this entire 4 measure vamp into other positions. The IV chord shape in the 5th measure is basically the same as the I chord shape, but with the E string lowered a half-step down (from 7th to 6th fret). Take a moment to notice that the flat 3rd of the I chord is also the dominant 7 of the IV chord. On the turnaround, we grab an Am/C (three note shape) built off the D, G & B strings, and then raise it a half-step in the next measure as a substitution for the V chord. Check out the backslide down into the I chord. Backsliding chords is a characteristic sound of jump/swing blues and something to experiment with using different shapes.