@HelenLockhart My best guess as someone learning Japanese, who has seen tiny bits of calligraphy but is very far from an expert:

一虫之 written vertically as

一
虫
之

I can't figure out any particular meaning or specific references to it online, though. "之" is reasonably common in names and pretty rare in actual words, so I'd guess it's some sort of name / maker mark.

(FWIW, the individual characters are roughly translated as 一 (one), 虫 (insect/bug) and 之 (archaic form of "of"))