The “tipster” source is unknown. However, the actual intelligence report(s) he reviewed, which contained the unmasked surveillance information, was simply President Obama’s daily intelligence Briefing(s).
It appears, most likely, someone (aka mysterious tipster), simply told Nunes which dates to review. Hence the Executive Branch SCIF was utilized. The ACTUAL intelligence product itself was the Executive Office Presidential Daily Intelligence Briefing.
Most likely a Clapper (ODNI), Rogers (NSA), Comey (FBI Counter Intel), and Brennan (CIA) work product. Remember, President Obama preferred written intel briefings which he reviewed from his secure iPad.
Reconciling the intelligence report that Nunes reviewed was actually the Presidential Daily Briefing (intel work product) also explains all the variant political concern angles.