The one on the left has a crimp around the case, which makes me think it's a smokeless powder load for a tubular magazine rifle, because the smokeless powder doesn't fill the cartridge and the bullet is seated on the crease. However that cartridge would also fit and be used in the the Colt revolver. The one on the right would be a revolver load also, for the same gun, but loaded by Remington, while the first one is a Winchester cartridge. The term Colt long wasn't an official term, just one consumers use to designate which cartridge they wanted. Note the shape of the exposed bullet compared to your find, which is what makes me think that's what your bullet is one of those types. The original cartridge was developed for the black powder 1873 Colt pistol. That type of pistol is still made today, just not by Colt, and in fact I have one in .357 magnum. Your bullet is modern, and possibly a low powered target load, or a spent cartridge that has traveled a long distance and ran out of poop. I say that because it's been fired but hasn't deformed from a hard hit. If it was shot into the water, it's possible it didn't deform a lot also, I know ballistics experts shoot into water to recover a bullet for testing against a crime scene recovery.
