Help Identify

This cartridge and ignition system I am unfamiliar with. To it’s left is a 44 WCF for comparison. Anyone that could explain the ignition system and guns it fits and it’s moniker will help.
It’s Dimencions are:
Bullet Dia 0.435"
Rim Dia 0.510"
Base Dia 0.465"
Neck Dia 0.465"
Case Length 0.585"
OAL 0.975"

Looks like a 12 mm Perrin,also called 12 mm French Tick rim to me.

It was intended for Lefaucheux weapons converted to CF ammunition.
Your round it’s inside primed

That heavy rim suggests that this is a 12mm Perrin, a Civil War-era handgun cartridge designed in France by (who else?) Perrin. These had an internal centrefire priming system sort of like the US inside-primed cartridges, where the firing-pin could crush the base against an internal anvil to fire the round.