It dropped last Iron Banner, and you can reroll it to get the same perks. It wasn't exclusive to the vendor version sold 2 months ago. The way Iron Banner weapons work is whatever was sold in the previous Iron Banner will drop in the next one as loot when you reach Rank 3. After that, you'll have to wait until it comes back around. It might have different perks as the standard, but you can always reroll to get what you want.