They're still in stock (no longer being produced). The board's eye logic doesn't support intensity settings, so it'll always read the bolt as paint unless you scuff the eye's surface. The manual for it is available on the SD page, including the wiring diagrams. Yes you need the cap to make it work, I'd go as far as recommending a slim 6800uF cap for the MQ to work more consistently.
Take a look at this thread: