Hi Rune,
Hymac produced the 580c between 1970 & 1981. I cannot tell the exact age of your machine by the serial number, I can give you a general guide as to its age, if the machine has cast Hymac nameplates on the
boom it is an early machine 1970-1975 if on the otherhand it has the Hymac name in big stickers on the boom it is 1975-1980. Sorry but thats the best guide I can offer you.
If you contact Richard Gould of Hymac he could probably give you the exact year.
It does sound to me that you have the wrong number anyway because Hymac serial numbers used to start with the machine type, for example 581001345 or 881000124.
Hope I have been of some help.
Jeremy