I found these pictures of the ex Littlefield Collection LT Mk VI
http://www.toadmanstankpictures.com/vickers.htm
In a couple of a pictures there appears to be a curved metal hinged cover more or less over where the bracket is situated. I wonder if this gives access to whatever is mounted on the mystery bracket. Might explain why it doesnt show up on stowage diagrams.
Forward of the cover is a semicircular fairing leading into the engine compartment. Could it be that this covers a shaft leading from or to whatever is mounted on the bracket. Could it be something like a starter motor, located there for ease of access??? Problem with that supposition of course is that British vehicles are not generally known to be user friendly.
Alternatively given its proximity to the turret basket ring, could it be for a power traverse gearbox or was the turret hand operated?
sk