This may be a shorted rotor BUT IT IS ALMOST ALWAYS an issue with the brushes/ brush holders/ brush holder insulators. Take a GOOD look at the brush holders. Both should have shoulder washers to insulate the brush terminal. If you take your time take them apart one at a time, and re--check continuity to the case with one brush removed you can find the culprit. Most of us can tell just by looking
Back in the day, we would NEVER replace a nearly new alternator just because of a brush. You can still buy brushes if you can find them.