I was random killing in the east side of the zone for about an hour, with a 10 minute excursion to the west side. Brass mouthpiece dropped in that timeframe.
This and the other three pieces (thin leather straps, bucket mute and polished bone horn) are for the Rogue Epic 2.0 quest.
Having said that, they are used for triggering the Reclusive Girplan ring event, but then he is almost always up anyway, so these are not really required.
However I think the triggered event is slightly easier than the untriggered.