Six-yearly, bi-monthly, every two months

Working on a magazine supplement this evening, I was surprised to read in the small print at the bottom of the flannel panel: "This supplement is published six-yearly."

The writer meant 'six times a year', but of course his turn of phrase could be taken to mean 'every six years'. So I changed the wording to 'every two months'. (It helped that I knew the supplement was published at regular intervals...)

One of my colleagues did suggest 'bi-monthly', but I decided not to use this as there seems to be some disagreement over whether it means 'twice a month' or 'every two months'. The OED Online concurs, saying "the ambiguous usage is confusing".

On reflection, 'every other month' might have been my best option. Not that it matters much, anyway - who reads the flannel panel?