One of the hypotheses about the earlier origins of the @ symbol is that it comes from old central markets. There @ meant “ at the rate of”, and was used to separate the quantity and the unitary price of goods:
Cotton 100t @ 2£/Kg
100 tons of cotton at the rate of 2 £ per kilogram
But we can dig deeper, does it comes from the latin ad, as a result of an a superimposed to a d with its “tail” curved over the a?

Or maybe its really a simple evolution of a late gothic a , like the Spanish arroba that made into a measurement unit ?

It’s difficult to know but it seems clear that its origins are plebeian and mercantile.

In English-spoken countries there’s little doubt about how to say @, simply “at”. But in other countries where they don’t have a tradition in its use, they have invented some funny ways of saying @:

update: Fellow noder Shark says that he never heard about "rouleau", but instead "arobase" or "at" for emails...