The cowl brace protects the brass radiator cowl from distortion brought on by the weight of the headlamp supports and front wings. Many have been lost or replaced with badge bars across of the front of the car.
The bar was originally flat section steel, bent at right-angles for fixing at both ends to ONE of the two bolts which attaches the headlamp stay on either side. You can see, in a curve to clear the fan blade, on the 65 being restored by Nick Boorman.
This one is original but presently upside down:
Here are some other designs adopted, presumably when the original has been lost, unless you can advise otherwise:
Rod Yates has some cowl braces made up that require bending to suit the inner cowl. Contact him for further details.