Just trying to help, not get on your case. It's not unusual for there to be confusion about the year of a Solex.
Besides the seven-digit frame number, there is also a separate seven-digit engine serial number stamped into the top rear of the engine, as far back on the engine as you can go, just in front of the steering tube. You may have to remove the engine hood, clean the area, and/or use a flashlight to see it. The engine number on your Solex will start with a 5.
Don't be thrown off by the 14-3-1966 date that you will see cast into the top of the crankcase...it is the date of homologation (approval) of the Cyclomoteur 3800 motor (March 14, 1966) and not the date of manufacture.