Part of the problem is that Lucas did not supply the steering lock assemblies on these cars. Lucas did make some of the switches as well as some after market replacements. My crusty old Lucas catalog does show the 39415 switch for 1970-1972 TR6 listed by TRF, there is also a handwritten note indication that 30563 works on later cars. Both of those switches are Lucas Model 157SA switches and the 30563 is listed for 1974 - 1976 Spitfire.
The 39911 (also a Model 157SA) is listed for the 1973 Stag. Given the "extra" wires and plugs on the 39911, I can't help but wonder if that switch would be the one used for those cars (including TR6 and Spitfire) with the seat belt interlock system that kept the car from starting if the seat belt is not hooked up. The 1973 model year sounds about right for that interlock system, but I'm not 100% convinced my memory is correct on that being "the year."