These old folks, you must know, were quite poor, and had to work pretty hard for a living. Old Philemon toiled diligently in his garden, while Baucis was always busy with her distaff, or making a little butter and cheese with their cow’s milk, or doing one thing and another about the cottage. Their food was seldom anything but bread, milk, and vegetables, with sometimes a portion of honey from their beehive, and now and then a bunch of grapes, that had ripened against the cottage-wall.
The above is from "The Miraculous Pitcher" by Nathaniel Hawthorne, derived from the Greek myth of Philemon and Baucis.
My question is that "seldom anything but" seems to be meant for "just only". It is a double negative form, I believe, and could it be replaced by 'nothing but'?
It is written in 1851, a long time ago, but is it a proper expression without any grammatical error?
I am not a native English speaker, so I've never come across with 'seldom anything but' kind of expression.