Full Series Fulcrum

I don't see any connection to Snoke or Rogue One, there is nothing to even hint at it. The original Fulcrum was always Ahsoka

Spoiler

(read the Ahsoka book), it was like her version of "Old Ben".

At some point prior or after her big reveal in the Season One finale, "Fulcrum" apparently became a codename for any operative to use thereafter as Ahsoka basically stepped out've the shadows and using the codename did not matter at that point. How Kallus learned of it, or if he did, remains a mystery but likely through the use of the Gall Trayvis agent as that was his(and the Empire's) contact agent within the rebel network/rebellion. Ahsoka always seemed to have used her facial tattoos as the Fulcrum signal(this was our first clue), how Vader never caught onto it but Kallus did is strange to say the least.