Anyone know where transponder gets its info from? Presumably if one altimeter only in your A/c , *that* feeds the Txponder, hence the need to keep to regional QNH en route.
There are (at least) two possibilities. Either there is a
blind encoder connected to the static system idependent of the altimeters, or one of the altimeters is an
encoding altimeter with the encoder built in.
In
neither case does the setting on the subscale of the altimeter make the slightest difference -- the transponder always reports pressure altitude from whichever source. In the case of an encoding altimeter, the subscale setting only changes the position of the hands, not the encoder output.