You won't find the express language you were looking for because the YS theoretically makes the X day disappear in order to award the trip.

A yellow slip is basically a "If you are going to use me on reserve, this is how I prefer to be used" tool. In this case, you are using the YS to "volunteer" to have an extra on-call day (see 23.T.2.c) at the expense of your X day or Golden X day and 23.T.6.b tells you that "[a]n X-day(s) lost as a result of such yellow slip award will be forfeited." In short, while on reserve you can either be on an X day or an on-call day but you can't be on both.

Because you are giving yourself an extra on-call day (up to a max of 2 per 23.A.59.c), you won't find the language you were looking for because that rotation you YS'd was flown over an on-call day, not on an X day. Again, that X day was lost and forfeited by the YS. Therefore, the YS rotation will only credit against the reserve guarantee just like any other rotation flown while on reserve over an on-call day.

A GS/IA, however, does not forfeit an X day like a YS does. That is why you get paid back that day and, because you were flying over an X day, you weren't flying over an on-call day and therefore you get paid over the reserve guarantee for that day.

Clear as mud?