Slot is index driven.

src/engine/devcmd/cmds/InfoCmd.java

@@ -229,7 +229,7 @@ public class InfoCmd extends AbstractDevCmd {
 
                         if (npc.getObjectType() != GameObjectType.NPC)
                             continue;
-                        output += newline + npc.getName() + " slot " + targetBuilding.hirelings.indexOf(npc) + 1;
+                        output += newline + npc.getName() + " slot " + targetBuilding.hirelings.indexOf(npc);
                         output += newline + "location " + npc.getLoc();
                     }
                 }