I found 70270 listed as the ID for the Boomthyr Rocket (Engineering, item 201014) in a WeakAura for tracking professions. I have completed the Boomthyr Rocket and that ID does show as completed for me, but I haven't been able to personally verify beyond that (though I have no reason to believe it's incorrect). /run print("Boomthyr Rocket:",C_QuestLog.IsQuestFlaggedCompleted(70270)) can be used to check the status if anyone else wants to verify.