1/2 pr stone
This rare kanji has no clear standalone meaning. The given gloss "1/2 pr stone" is obscure and likely refers to a specific historical weight or gemstone measure. Treat its meaning as uncertain.
玗 combines 王 (jade/jewel) with 于, which likely contributes the sound. The exact historical development is uncertain, but it is probably a phono-semantic compound related to a type of jade or precious stone.
The left side 王 suggests a jewel or precious stone, and the right side 于 looks like a hook or a measure. Imagine a small jade weight used on a balance scale: a half-weight stone.
For ウ, picture a jeweler saying "ooh" in admiration at the rare stone: "ooh" -> ウ.