The weapon was primarily used not for "lighting up the sky", but for law enforcement and counter-terror units. A specific round developed for it was a barrier-penetrating tear gas round ... supposed to be able to pass through an airplane fuselage or bank window. If you're using the flare gun at night to hit a window, some form of basic night vision might be useful.
The two holes are threaded, and there's a tiny ... detent pin? ... along the bottom edge of the slot. The slot itself is about 40mm long, 5mm wide (measured with a ruler, not calipers or a micrometer).
Thanks for your attention!
EDIT: the high-velocity, possibly rocket-boosted wall-penetrating rounds (max range 700 meters -- across an airport) aren't for use in regular flare guns, which is why the RV85 has a really thick space for the cartridge rim -- the "rocket rounds" have an odd pressed-metal "staggered" rim, so they won't chamber in regular launchers. You need to put an bit rubber O-ring on regular flares for them to fire reliably from this flare gun. I've attached a pic of some rounds.