This place gives some info. I don't know how accurate it is.
[http://www.audiotubes.com/mullcode.htm]
[http://www.audiotubes.com/mullcode.htm]
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?