GIBSON sold the STYLE U HARP GUITAR between 1902 and 1939, although most were probably made before 1924. The Style U was the largest of the range of Gibson harp guitars with an overall length of 48" and width of 21". Initially the Style U had 18 strings - 12 sub-bass harp strings and 6 conventional fretted strings. The scale length of the guitar neck was 27.25" with the maximum scale length of 35.25" for the bass harp strings. Later models had a more conventional 25.5" scale length for the guitar neck. The number of harp strings was also reduced to 9 or 10 in later models - although it is not uncommon to find that extra harp strings have been added as a modification on these Style Us.
A fancier version of the Style U was the Style U1 with intricate fingerboard inlays.
Source: Gibson harp guitars at harpguitars.net