Is the carb dated Dec 66 the one with the spring on the throttle plate by chance?
What is of interest is that I have seen both types but never compared them to the months they were made until now.
Maybe this thread stumbled onto answering why there was a differences and period of change?
I suspect that (with a cold engine) the spring design was more of a positive method of controlling the secondary circuit. By that I mean keeping the throttle plate shut, there is less chance of gas flowing past the secondary needles, even when they are seated, due to hydrostatic pressure differences between the secondary nozzle tip, that would otherwise be exposed to vacuum if the throttle plates opened, and the fuel bowl as would be with the other design type of keeping the secondary butterfly valve shut (non-spring version). Just my guess.
I would use the carb closes to the date your car was made based on the trim tag date stamped.
I looked at both my mid-year L35's (4B w/AIR & 5B non-AIR) and they both use the spring type of secondary throttle plate.
What month was your car made?