So, what I have learned so far:
(Thanks for your input Ian G.
you inspired me to start the hunt)
At some point, the Boler
serial number did became 6 digits (Probably 7 digits since there are 12 months in the year).
The first digit is the month of production: 3 = March
The second digit is the year of production: 4 = 1974 (would 1984 also use a 4?)
The last 4 digits is the production number: 0666
What I do not know is if the production number is for the month, year or total production from some point in the past. For example, do they start over again at 0000 (or 0001 or some other number) at monthly or yearly intervals?
666 units in one month may be possible, but unlikely.
If it is the total for the year, they would be averaging 333 units/month with a yearly production of over 3,900 units/year. (this is all speculation so anyone that can spread some facts over this guess work would be helpful!)
Post up your S/N and lets see if we can see a pattern!