By when it’s made. If it’s after whatever the date was that MSD bought out Mallory then it’s an MSD box with a Mallory sticker on it.
It’s sad, because MSD could have kept just two Mallory boxes (the 685 digital 6 box and the 667C and S boxes which are 7 boxes) and scrapped all the MSD line except for the grid.
That would have made sense. And, when Holley came in and jacked everything up, they could have killed every Holley pump and regulator and just used the Mallory stuff and they would have been far, far better off.