Old methods where the quick solution was just copy the whole data without take attention about his structure 
Actually I know this packet as “CLIENT_INVENTORY_USE_ITEM = 0x704C” and like his name said, is for using all kind items.
byte slot
ushort UsageID
uint UniqueID (optional) if is like a potion or pill for mount and pets, in that case you have to specifyIn python words:
Packet = struct.pack('B', slot)
Packet += struct.pack('H', UsageID)
Packet += struct.pack('I', UniqueID) # (Optional)