It's a little weird because in '67, the court redefined the 4th around the idea of expectation of privacy (Katz), and Miller and Smith followed from that redefinition. So you might have happened across something relevant under the previous scheme, like Boyd in 1886.

It was a customs search case from the turn of the 19th century, but now I'm wondering if I got my wires crossed between that and border search.

Probably. Border has a lot more from the original congress, and more also about customs since that was one of the original boarder exceptions.