[T3] Reverse lockout

[Larry Thrasher]

I have a 1968 Squareback. The reverse lockout does not work (i.e. you can shift into reverse without pressing down on the shift lever). It shifts into all gears without any problems.
The lockout plate on the '68 has two 'tabs' and I have it installed according to the Bentley Manual. 
I did notice a groove worn into one side of the plate. Is this the source of my problem? 
Thanks.