Well I ran this through my gray matter for 48 hours and barely resisted the temptation of Google.
Never would have guessed Leyden jars. Never. Like others above I was thing jars as packing containers for safety, sparks, humidity, shelf life, and on and on. None of it seemed right though and without EVER seeing any NOS on eBay for capacitors packed in jars in "Naval minty" condition
, I knew I was running in the wrong circle.
Unit of measure. Approximately a nanofarad. Whoda thunk it
? Great quiz question, Carl.