What do the numbers '3-5' in that name refer to if it's version 3.6?

There was never a Sonnet 3.6. They released what is commonly known as 3.6 as "Sonnet 3.5 (New)". Then, because so many folks ended up referring to it as 3.6, they decided to call this new model 3.7, as the mental territory for 3.6 was already occupied by 3.5 (New). Not confusing in the slightest!

Under unavailable I meant unavailable. I was talking about 3.6 not 3.5.

There is no 3.6. There is 3.5 and 3.5 (New), both of which remain available.

Ah, you are right, I found it (it was under the more menu on the UI).