Amperex tube codes... how to determine year?

If the 3 digits after the delta indicate year, month and week, how do you determine the decade? For instance, delta 3A5 could mean 1963, 1973 or 1983. Does the 3 digit prefix before the delta indicate the decade? For instance, is ID3 earlier than ID4 and so forth?
This place gives some info. I don't know how accurate it is.
This can make someone crazy...LOL. Another to look at.[]
ID is the tube type. 3 is the change code or version so, yes, 3 is earlier than 4. When enough changes had been made to the tube to consider it a new version the change code was moved up.

I think the Heerlen (delta) plant made tubes from the mid 50s to the mid 70s. Tubes from the 50s had a 3 character factory and date code, delta 8A would mean that it was made at Heerlen, in 1958, January (Jan = A, Feb = B, etc.)

In the early 60s they added another digit to the date code for the week of the month. So your delta 3A5 was probably made in 1963, January, week 5. It could also have been made in 1973 which you could tell by structure (dimpled getter) or color of paint used for the label (orange or green instead of white).

If that and the link Hifvn gave you don't clear it up, let us know.