I've got to ask , is that top plate back to front?

I'd say so... and I'd be careful with the lagging on that support!

Where the magnetic arc deflector was located? Above or below the reflector?

There is no arc deflector in this lantern. I turned it on its end to run the MA/V lamp. If there was a deflector fitted, it would be above the reflector. The plate is not back to front, that hook is in that same spot in all three of my Horizon lanterns and the others seen in collections. Not sure why its there as the base seems strong enough on the lamps to hold its own weight, unlike SOX. This lantern is full of asbestos, that sheath on the support, the gasket, and most wires are all the white stuff. It does look flaky so I'll be removing it once I have a few more masks and the time to do it.

If it fits tightly around the lamp neck then it might be there to dampen vibrations and prevent lamp disintegration (i have read in James' site that this was a problem with the spot weld assembly of lamps of the year range when this lantern was made). Alternatively, all the examples mentioned in people's collections came from the same batch and are indeed all back to front

The support wouldn't fit an elliptical lamp if it was at the other end. Plus the catalogue page on Simon's site shows the hook in this place. The anti-vibration theory makes a lot of sense

The chamfered ends of the reflector plate also look like it is meant to be this way round too