Here's some more sources and info. In the 1971 Gun Digest a Beretta SO2 is listed at $950. As Ron stated above it was the entry level into the SO series line. Basic difference between the SO2, SO3, SO3EL (SO4 in the Garcia line), and the SO3EELL (SO5 in the Garcia line) was engraving, better quality wood, and hand detachable locks.
Besides the RL Wilson book on Beretta, there is also "The Legendary SO-Beretta" by Gianoberto Lupi. This book provides more background on the SO series but is really about how the Beretta SO is the end all be all to the O/U design.
You can try to find a definitive source on the sidelock pin design, but other than the thread referred to by eightbore nothing is documented. Lupi's books states the sidelock design changed from the basic H&H type of sidelock (7 pins) in late '59 or early '60 (5/4 pin).
Early SO guns are great game guns as they are usually around 7lbs. Later models became heavier especially into the late 70's.
