I have never seen much correlation between the various changes/evolution that has happened and the model numbers. Boehler Antinit steel seems to be common until quite recent times and has a reputation for being excellent for corrosion resistance.
I have seen a number of variations of the pin placement on the locks, so clearly there have been various changes there.
You are lucky to have double triggers and straight hand stocks. Most guns here in the UK have single triggers with a very bulky 'pistol grip'.
SO5 was a plainly finished competition gun.
I have an SO6, which is a game gun, and has quite light restrained scroll engraving, whereas the SO6 EELL has much more elaborate engraving available in game scene and scroll variants as well as a 'custom' option.
SO9, SO10 have different closure/locking as has been noted in the thread. In the UK, SO9 and SO10 were only available in 20 bore for a while - and I had heard, though not sure if this is correct, that manufacture of SO9 was outsourced to Abiattico and Salvinelli.
All of the SO series have become eye-wateringly expensive as new guns in recent years. (They were never cheap, but seem to have increased more than other Beretta models in the last few years)
