What you see is obvious. This is simply an inverted U-shape bracket holding a hinged plate. The object is to prevent accidental override of the standard spring loaded reverse lockout when downshifting from 3rd to 2nd in the heat of competition.

On 7/31/2014, Edward Vandyk in the UK wrote:
"I was looking for a reverse gear lock out for the replica of Bob Olthoffs car I have built, As an alternative to the hinge I think this might be better. I am not sure it has been done on an MGA before? The chrome ring is MGB".


Addendum March 22, 2022:
There are many minor variations on the schemes shown above, most being visible on top of the tunnel, and most requiring some time to pull a pin for throw over a hinge before allowing shift into reverse, none of which I would like.
By the mid 1960's I was learning to drive on a 1947 Ford heavy duty pick-up truck with 4-speed crash box (non-synchro) sliding gear transmission. 1st gear in that one was a "granny" or crawler gear for slogging around (slowly) off road, but almost never used while driving on road (normally start off in 2nd gear). There was a reverse lock-out mechanism. I would have to search to find how the mechanism at the bottom worked, but the part up top was a simple thumb trigger and pull rod just below the shift knob, the only two visible moving parts. That was simple and appealing.
Now Malcolm Cox in Oceanside, California USA has a reverse lockout that can work as quick with one hand in a second without looking, and I like it. This one has a neat sliding concentric sleeve on the shift lever rather than a pull rod on one side.

Right now it's 3D printed which will be upgraded to metal before it sees action.
Malcom also wrote: "Here's my just finished shift boot, mounted on experimental reverse lockout, no holes drilled in sheet metal, 3D printed rings, and Amazon $6 vinyl boot (and a lot of fiddling)".