I thought the Txpdr alt was fixed at a datum of 1013, from a separate source ,and independent of whatever the crew altimeters were set to..
I think some confusion is coming up.
Of course, any aircraft will send the alt signal (mode C) in relation to standart alt setting.
Here we are talking about mode S, and what a controler can see when interrogating the setting. That obviously differs on some aircraft from the 1013.
Nic