I do have "Preferences - Behavior - Preferred output format = ePub". That doesn't seem to be the decisive factor ...
And the output is the same whether I use "Send to device" or "Save to Disk".
Release: 0.7.25 [29 Oct, 2010]
Add support for the SONY periodical format. This means that news downloaded by calibre and sent to a newer SONY device (350/650/900) should appear in the Periodicals section and have the special periodicals navigation user interface
Good catch! That's exactly what I'm trying to activate on both notebooks.