According to Paul Skilleter's book chassis numbers 1005483 to 1051385 were produced in 1963.
In Ray Newell's book chassis numbers 1043218 and 1043752 were produced in October 1963 so by a process of elimination I'd say chassis number 1043509 was probably produced in mid October 1963. It could even be it's birthday today
Just curious as I'm looking at bidding on an eBay junker for parts and he reckons it's a 60. It's on an A plate but it's age related. My 63 4 door is an August build and is chassis number 1032NNN. I knew it was later than mine as it has the elephants trunk for the heater and mine is a smiths.