The letters and numbers preceding the six-digit number are actually written with the first letter over the other two. For example, the Guinness Extra Stout is "L over A1".
A: Take the first two numbers and add 11. This was the year the batch of beer was brewed. The remaining 3 numbers stand for the day and date. Guinness stopped the date labels some time in the 1990s.
however,,,
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In 1949 Guinness began brewing stout in the taken over E & J Burke Brewery on Long Island, New York. Examples of labels exist showing adherence to the conventional code, i.e. N/50 500822, 22nd August 1950. Examples of L, M and N exist on these labels.
In 1956 it was decided to alter the code so that the batch date could not be easily deciphered. So L/A10 702112 was in fact 1st October 1959. It is unclear as to the meaning of A10 (possibly a bottler reference) but the date is read by dividing the code into pairs of numbers and deducting 11 from each pair and then reading in reverse. The earliest such date code found is L/A5 691612 from 1958. Examples of the A(number) beneath the L range from A1 through to A15.
Given this I think it is 1974
"A Bottle of Guinness Please": The Colourful History of Guinness - The book
Tooheys bottled from 75-86 but usually use the number 86 at the start
If it was SAB they they bottled from 64 - 75 and used a different code for each year.. so maybe it is 1974 !
cheers