The only digits I have totally ruled out are 1,2 and 4. There is still the possibility of any other, as it has been disfigured by damage, decay and eruption. I have not found any notes anywhere about this. There will be no more railing at the car.
If I am correct according to Alverton: West of Penzance (A History) www.paulmason.info/Alverton Alphington House and The Weeths in Alverton Place were both built about 1830 and could help with dating this post.
I do believe you're right in location CW! I posted a section of it (a while back now) but managed to resist the temptation of steaming in and nabbing the points when I saw this!!