While I think you are probably correct, this is where the SRH example #6 gets weird. In that example, the duty day for the REG pilot on day 4 didn't even get extended, in fact it was shortened. Yet L8 is paid for that day 4 duty period.

So it appears that for a REG pilot, only the footprint of the rotation needs to extend beyond >4 hours for L8 to kick in, and that L8 pay includes the duty period where the reroute began, regardless of the end of that duty period. But for RES, L9 only pays as you say, for duty periods that both end >4 late, AND infringe an X-day.

That's a massive difference that doesn't seem to be represented in the language of the PWA, the SRH, or any scheduling alerts.

Both L8 and L9 include the phrase: "single pay no credit for any duty period(s) that extends beyond such time limitation.". SRH example #6 gives L8 pay to a REG pilot for a duty period that didn't extend >4 hours (day 4 of the rotation.) This suggests that the duty period itself going >4 hours ISN'T the trigger, rather the footprint of the rotation being rerouted >4 hours is. And if that's true for REG, I don't understand why it wouldn't be true for RES if the footprint of their rotation is rerouted >4 hours and into an X-day.