It wasn't an engram weapon in yr1 but xur still sold it, i didn't get in in yr1 so 3 months is a lot less time.
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Xur has never sold thorn cause it has a always was a bounty weapon so you could never get thorn from xur unless it was from a legacy engram which was in y2
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Just do iron lord bounties to get jagged purpose. And I don't think xûr sold it in y3