Novatron "N-3000 Digital Flashmeter" help?
What an old beast. Mallory 9V or equivalent battery. Mallory/Duracell, Duracell has been through several owners since then; Kraft/Nabisco and now Proctor & Gamble.
I'm guessing the meter is from the 1970's with that red LED display reminesant of the early digital watches. Shutter speed either only 1/60 or 1/125th sec. I'd be a little leary of the accuracy of that old thing, probably can't be calibrated and is way off. You're probably better off using a grey card and the histogram on your camera than this meter. Newer Novatron flash meters used a simple LED scale and a graph/chart on the front of the meter and were better off used for film than digital.
Otherwise the controls look straight forward and simple. The lever on the sensor is probably for switching a diffuser in and out behind the dome for reflective/incident flash readings. The manual button probably fires the lights when connected to the PC connection next to it when the meter is connected with a wired sync cord to the lights.
If you need to use a flash meter, I'd recommend you scrap this one and use a more modern Sekonic, Polaris, Gossen, or Kenko/Minolta meter as the Novatron you have may be frustrating to say the least.
I know, not what you wanted to hear,....but. Besides, you'll have an easier time finding an owners manual.