The Martin GT-75-12 was the 12 string version of the GT-75, made in 1967 only. The GT-75-12 was Martin’s only 12 string electric guitar. To fit the additional tuners the GT-75-12 has a elongated headstock - of different style to the GT-75 and GT-70. Evidently the low sales of the GT-75-12 the meant that some of these oblong headstock necks were surplus to requirements and were used on 6 string GT-75s. All GT-75-12s had red finish and the Bigsby vibrato was not an option. Although Martin factory archives only noted three of these 12 strings they were made in greater numbers than this.

Source: A Marshall - Martin Electrics