An ES2 mag must be the same rotation as a 16H. "Clockwise at the drive end" The only difference might be, is that a 16H has a tight wire advance (cable at front of points block nearest cylinder) I guess by 49 it was slack wire....But will work just the same.
Ron, as far as I'm aware, your comments apply up to the end of 1947 but as a result of the 1948 redesign which ended the use of lever followers, the inlet cam was no longer driven as part of a train from the ex. cam. This means that it turns anti-clock.
The differences are all within the cam / points housing so can be changed. Mounting plate fixings should be the same (but mouting plate overhang is not). WD and early Nortons had the HT pick-up next to the cylinder. I'm not sure if there are any practical reasons for this, or if the 1948 onward machines also featured this.
Yes, indeed but so many mags have been put together from bits that you reall have to check everything. Don't rely on the arrow on the end cover or the spec. plate as they can be easily swapped.