Gibson has used the "Open Book" headstock design since their inception, and while the design did not originate with them, they became synonymous with it and later trademarked it as their own. It has seen many changes in shape over the years, and knowing these changes can be helpful in learning about your SG.
Comparison[]
When it comes to modern Gibsons, the main three styles are Wide, Medium and Narrow:
The Narrow style is mostly narrower at the bottom end. This is the easiest way to tell one from a Medium. Conversely, the Wide style is mostly wider at the top end compared to a Medium. It also has more pointed corners at the top. The narrow headstock also has the tuners spaced closer together than the other two.
The SG debuted with the medium headstock in 1961, and switched to a narrow one in 1969 with the crown inlay positioned lower. In 1971, coinciding with the SG's redesign, they introduced their widest ever headstock, with very little tapering at the waist. In 1973, they increased the waist taper and narrowed the bottom end. This was reversed in 1977, going back to the wide headstock. That would be taken and narrowed at the waist and the bottom again (as well as returning the crown inlay to its original position) to be used on the 1986 SG-62 and was subsequently kept for the '61 Reissue. Meanwhile, the modern narrow headstock was first introduced in 1982 on the SG Special and SG Firebrand Deluxe, before eventually spreading to the Standard in 1991. Ironically, the narrow headstock was originally created as a more historically accurate feature for the Heritage Series in 1980. Only with the introduction of the Historic SG in 2000 did they begin offering a true replica of the original medium sized shape.
Internally, the narrow Standard headstock is denoted "SP1" and the wide '61 Reissue headstock is denoted "SP2". As SG Customs are also denoted SP2, it could be that the wide design originated from the extra width added to the headstock by multi-ply binding.
More recently, a new version called SP-1 B came out, being slightly wider and with slightly pointier top corners than the SP1. You can differentiate this from an SP2 by the fact that the SP-1B has the crown inlay location and tuner spacing of the SP-1.