The standard Press 40 flashbulb is pictured at left. It is distinguished by it's "Edison" screw base, just like a normal lightbulb. The bulb is smaller in diameter, however, and only about 3.75" tall.
Like all flashbulbs, it's a one-shot deal... Once you fire the bulb, it's cooked. Quite literally. Bulbs have been known to break and often the glass melts in places. So use extreme caution handling these bulbs.
In old movies and documentaries, or any film depicting the early 20th century, you'll see photojournalists using these bulbs on their 4x5 press cameras and often using a handkerchief to unscrew the old bulb.
You will find unused bulbs randomly in flea markets, old photo shops and on eBay, but they tend to be far more rare than the smaller bulbs that later replaced these - like the Press 25 bulb.
If you have trouble finding Press 40 bulbs for your flash unit, you can also occasionally find an adapter that features the screw mount topped with a bayonet mount for various other bulb types of roughly the same power. These adapters are also a little rare and can seem pricey - but when you can buy lots of another type of bulb, it's worth having an adapter.
Do you have experience using these bulbs in your box camera? Any helpful tips? Have you seen a stash somewhere? Do you know some trivia about the history of flashbulbs or Sylvania or the girls on the boxes? Add your stories below and the best will join the official canon.