CF Terminology....Plate, Mounting, Counter-Weight, Useless, Mk.1
Just kidding...there is no name for it. It was essentially a counter-weight of sorts mounted to cover the hole created by not mounting the British sight. On the Scorpion and Scimitar a BAE Systems 5.8 X magnification passive night sight was mounted there as the turret sights were only day sights.
Note the passive sight mounted in this Scorpion image...
The only spotlight on the vehicles was the Alvis spotlight that could be hand held or post mounted and plugged into a socket on the side of the left grenade discharger. (You need to move your power cable to the other side of the turret)
There are a few nations that opted to not have the night sight mounted and also have the same cover in place on their Scorpions...Ireland and the Philippines for example.
Of an interesting note when AVGP competition was being run in the 1970s Scorpion turrets were fitted to other AFV hulls and not just the MOWAG contender. The MOWAG contender with the Scorpion turret used for the competition did indeed have the passive night site fitted to the turret. As well, the MOWAG contender hull did not exactly look like the final AVGP hull...there were some differences in hull angles and the external exhaust.