I believe they've all been convertible since the mid-sixties.
I believe there is a groove in the breech-face of the Long Rifle cylinder, but the Magnum face is smooth, but without going and digging one out of the safe I'm not absolutely sure. The really best way is to remove the cylinder and attempt to put a magnum in it. If it fits, it's magnum. If it won't, it's long rifle.
Ruger will sell you the cylinder. At least they used to. But they won't just "sell you a cylinder". You send them the gun and they will fit a spare cylinder to it. Barrels and cylinders are "factory fitted parts".
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I always take precautions
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