If setMaskBit was good enough for lure of the wildcat (80 NPCs, 20 per town), it is good enough for stamp hunt (7 NPCs).

I was seeing errors during stamphunt. Sometime it worked fine, sometimes a character would hit all 7 NPC's get all 7 messages saying the "received a stamp!" and then the quest wouldn't finish. Now it work without any chance of error.