This is a classic British all-brass flare gun from a design of around 1900. The design was in use even in WW2, although the Brits by then made it out of pot metal, rather than brass. The Aussies, however, must not have gotten the word, because this Austrian version, made in 1942, is still brass. Brass is very good for shipboard use. It not only resists the marine environment, it's something extra the swabbie has to polish.
As shown next to the dummy Glock 21, it's not a small item. It fires standard one-inch pyrotechnics. It's also got military and manufacturer markings all over. The Brits and the Aussies are almost as big into markings as the Germans. Here's one big batch of markings, although there are others in other locations, too.
Sean Connery used one of these flare guns to take out an enemy speed boat in "From Russia With Love", along with shooting down a helicopter with an AR-7 survival rifle.
Well, he didn't actually shoot down the helicopter, but wounded the guy with the grenade, which then rolled around their feet. As the saying goes, "after you pull the pin, Mr. Grenade is no longer your friend".
Perhaps that's why even flare guns are illegal in England, unless officially deactivated. And while they're also cracking down on pellet guns and knives, soviet-bloc weaponry is flooding the country, courtesy of either organized crime or TROP. The bad guys have AKs, the cops their issued MP5s, and everyone else has to duck.
Even modern pyrotechnics are frowned upon, so boaters can't take advantage of the current batch on non-gun plastic 12ga flare guns and shells. I guess they'd frown even more if they saw my rocket-assist parachute flares. Of course, I have to remember where I put them, first.