Looks like one of the flanges for the wastegate. IIRC, the ADC kit has the wastegate positioned on the uppipe to where it sits right by the downpipe to dump into it.

So, I'd bet it's the exhaust port flange to the wastegate.

2005 Ford F150 SuperCab FX4 1964 Chevrolet Impala SS 1998 CSVT: 354HP/328TQ @ 10 psi, now gone