But why did you rename all the variables (ie from karte_t::get_finance_history_year() to karte_t::get_world_history_year) ? Imho the old name is much more descriptive than the new one, which contains even redundant information (ie world, convoi etc).
With the renaming, you would know, what a history you get (since all histories has different entries). The old name get_finance_history_* wasn't better, imho. Renaming it to get_history_{month,year} shouldn't be much effort...
Maybe I got 'inspired' by something like
bev += (*i)->get_finance_history_month( 0, HIST_CITICENS );