The blueprints call out 3/8-20 (BFT) thread pitch, I could not find BFT anywhere. What does BFT stand for? (British Fine Thread)? I found some 3/8-20 Taps on Ebay which I purchased and came from GB. They are 60 degree thread pitch angle, not 55 degrees which is Whitworth. The 60 degree pitch seems to be correct with all the investigation I have done. The taps match the pitch on the stud and the nuts I have made and tapped with the taps I ordered match and thread on perfectly. So, IMHO, 3/-20, 60 degree thread pitch is correct for the Lucas Square Eight fog light stud/nut.
Note: When the book was written, it was the belief of the author it was Whitworth thread as the lights were from England. My investigation shows differently. Hope this helps. Mike