The pin design and S/SO designation really are a bit of a mess. For example, I own an S3EELL, made in 1986, 3 gold plated pins on left, 4 on right. In a letter to me, Ugo Beretta verified the date (also checked with proof marks), game scene engraving and engraver, etc. It was sold to a gun shop in Rome originally. I've seen others of about this vintage for the American market designated as SO.
The basic gouge on the pins is that the fewer, the more modern (and allegedly improved). All of the Garcia period models I have seen were designated as described agove, and have 4 pins left, 5 right. Often these guns have detachable sidelocks with the clever hidden (mostly) thumb levers.
The SO4s and SO5s of target get up (including current SO5s) are a different pin configuration yet - 4 pins on both sides.
