No it's right next to a flatter screw used for a ground and it isn't very big, which is what confused me. In recent response to another n00b with similar issues on a Mk2, T3 Ragtop described it as a "large screw on the back of the Radio".
Might be on the Mk2, but on Mk3 it is not so large and it's actually facing upwards on the bottom of the cross member nearer the center than the ground screw and with a smaller, rounded head. Found another post where somebody else had the same trouble interpreting a similar description.
Here we go 'round again

At least I got the stupid thing out without breaking anything.