If memory serves correctly, they boiled (to sterilize) the jars. When cool enough to handle, the jars are packed almost to the top with butter, and salt sprinkled on top. I don't know how much, but at least enough to cover the butter --probably 1/8" or so? Then put the lids on to seal the jars. You do NOT boil the jars to vacuum seal them, as you would when canning fruit/veg/meat --just screw the lids on. Evidently, the salt, combined with the hand-sealed lids, is enough to preserve the butter.

I think it's from the Hiscock book "Come Aboard" (which I read this past summer). It seems like a great way to keep butter, like the tried and true method of greasing farm-fresh eggs to be able to keep them for a long time unrefrigerated.