If it was tossed aside I would think there would be a reason for it.
But as Truth said, burst discs have different threading. Also, you never ever want to remove a burst disc period unless it blows. Burst discs need to stay in their one specific port. It's a major safety feature and even swapping the LP and HP bursts discs by passes a huge safety measure; making the tank potentially dangerous to fill.
As far as the nipple and gauge, contact the manufacturer and do what they recommend. Sometimes they do not want you to remove anything and always go by what they say. If they say you can swap the nipple and gauge, then go ahead.