When I purchased my GT/CS, it had the square Lucas mounts also. My car was built on February 14, BUT was not sold from the California dealer until May 15th. Remember, in California, these cars had to be retrofitted with the Lucas lights, and this occurred around the end of April, 1968.
It is my opinion that the replacement Lucas lights came from England as a complete set with the square bases, and the dealers used the chrome square bases instead of the "correct" tubular mounting brackets when these lights were retrofitted at the dealerships. It only makes sense that the dealer would use the Lucas square mounts that were sent with the replacement lamps. It would be crazy for the dealer to order the tubular mounting set AND the Lucas lamps (two seperate orders) when mounting bases were already included in the Lucas lamp set.
Once the Lucas lights were standard equipment at the factory, I'm confident the factory installed the "correct" tubular mounts on the cars.
I have this opinion because there are just too many original GT/CS cars with the square base mounts, and this can't be rationalized as a simple error.
Just my thoughts,
Russ