The Solex 40 PII NDIX Carburetor was introduced on the Porsche 356 with the introduction of the Super 90 engine in 1959. These carbs employed a solid shaft and were used on the subsequent 356SC engine (as well as the 912 series).
Jim Kaufman at Carburetor Rescue shared the following: Because the Solex factory made different runs and maybe farmed out some of their stuff, you can't put a whole lot of stock on the actual number. Early S-90 carbs had a crude sort of stamped number, while later types for SC's had more refined numbers. Usually these numbers were 4 digits long, but some had 5 digits if you will notice in my set. In '68, when the factory used the vacuum advance distributors, they were 6-7 digit length starting with an 8xxxxxxx meaning, he thinks, standing for the year '68. But then in '69, the factory went back to 4 digit numbers which started with 7's thru 9's. In a nut shell, there is no precise correlation of serial number to year.
When asked what was his favorite Solex, he said the 912's with the improved internal circuitry (independent idle vs dependent idle circuits) and superior bypass port numbers (5 instead of 3) for better low rpm to high rpm transition, were the overall best. BUT, because split shaft carbs wear faster, and consequently don't work, the best solution is to have your split shafts converted to single shafts with bronze bushings - the best of both worlds.