Throughout Gibson's history, while they have mostly stuck with their "Open Book" headstock design, there have been many changes in that design over the years. Knowing the differences can help you date and better understand your SG. In modern day, there are three main styles; the Standard style, the '61 Reissue style, and the Historic style. Generally, this suggests which basic model any limited edition may be based on (in conjunction with the body design).
The Historic style is what was used from 1961-1968 and returned with the Historic Collection in 2000. The Standard style is similar to the short-lived style produced from 1969 to mid-1971 because it is the narrowest and has the crown inlay positioned lower. The biggest difference is that the modern Standard style tapers to the bottom while the 1969-1971 style did not. The '61 Reissue style is similar to, or the same as, the Norlin style introduced in mid-1971, being the widest and having the most protruding pointed edges.
These headstocks are pictured below, going from widest to narrowest (left to right):
