It was the time in which our du was rooted, the time when he too must have called me by my Christian name, I hear it no more, but it is unthinkable that at six or eight years he should not have called me Serenus or Seren just as I called him Adri. The date cannot be fixed, but it must certainly have been in our early schooldays that he ceased to bestow it on me and used only my last name instead, though it would have seemed to me impossibly harsh to do the same. Thus it was -- though I would not have it look as though I wanted to complain.
In Mann's extensive communication with Herman Hesse, whose The Glass Bead Game unsettled him by what he saw as its resonance with Doctor Faustus, and with whom he grew closer and closer over the years, neither Hesse nor Mann ever use anything but the formal address and never the first name only.
I picture Mann writing his Doctor Faustus on a hard wooden seat.