I put a early 750 tail light (known as the Beehive) on my Mk 2 850. It was a pattern fibreglass thing made in Taiwan or India. It's crap. If you use a genuine Lucas Beehive, you need the upright bracket, as the 750 bolt positions are different and it will sag down.
These pattern copies have bad wiring, the earth wire was useless, I ended up fitting my original 850 bulb holder with its solid earth point and fitting proper gauge DC wires so I at least had a relaible tailight.
I also ended up extensively modifying the pattern part, it cracked, flopped around and generally was a pain. It needs rubber grommets on the 2 lower side mount studs and reinforced expoxy on the upper mount to get it central and symetrical to the mudguard. I have now literally remade it and it looks OK, but otherwise more trouble than it was worth.
Mick