Hi Bacho & Elcazador,
sorry, it took me some days to follow op your kind replies.
Anyway, thanks for your tips!
Looks like my inter 20 is not the only one that suffers from widened mast tang.
But- drilling the old rivets repairing the tang outside with a much thicker vertical plate would forecome these troubles once and for all. Do you know the mast manufacturer, maybe they can tell which rivets needs to be fixed and if it would be wise to proceed in this way....

Norbert