I think from your description that this is in fact a decanter by another Swedish firm, Kosta.
The number 558 is a re-issued design by Elis Bergh from the 1930's that was re- introduced in 1959. The three rows of waves are in fact internal (that is on the inner surface) horizontal ribs. The stopper should also be solid sphere and peg, not hollow.
I believe you will see a sort of indistiguishable incised mark prior to the number that might approximate a "4" or a "K", which should clinch the deal!
Kind regards, Nigel