A number of European airlines have a variable MTOW amd this is declared at the start of the flight and they are charged the airways fees based on that MTOW, rather than breaking the rules a PA34 owner might like to apply for this arrangement and stay within the rules.
Sadly I do not fly Senecas nowadays but it was a practice that went on and still does.
What you are saying above is very interesting and I never knew it was possible to have a variable MTOW on an aircraft like the Seneca.
Would this mean carrying two flight manuals? One for above another for below?
Pace