AScot and Faldrax have replied to duplicate copies. I will continue with Faldrax's reply and Charles can combine the two. He is cleverer than me
I cannot fathom out the naming convention Sony (and the other mobs use).
We have A100, 200 etc and (my first Sony after the Minolta 5D) 700, 900 and then we jump back to 850.
Then we get the SLT's
33, 55, 99 (OK 99 rolls off the tongue but then if a new model arrives where do you go?)
After the NEX which I never got my head around, we have
A6000, a6300 (what happened to 6100 and 6200 ?)
Then we have the A7 (up from NRX5 6 suppose)
Now we have 7, 7r, 7s, 7ii, 7rii, 7sii. why?
At least the lens designations have a modicum of sense and if you see an sel55 or an sal300 you have a rough idea of what it is.