Siscia Mint

What does BSISC on a Valens Roman Coin stand for?

I think the SISC stands for Siscia mint but I’m not sure about the B.

You are right about Siscia, I believe the B would just be the officiana that minted the coin. In the later empire, to have even stricter controls of mints, they subdivided the mints into officianas, or submints, so that defective products could be more easily traced to makers. On some coins the officiana is in the field of the reverse, on others it is incorporated into the mint mark.