Aluminium 2S-O

[Gadgetmark]

Can anyone tell me where I can find reference to Aluminium 2S-O? I understand this was the pre 1970 way of expressing aluminium alloys, now we use the 1XXX, 2XXX, etc international system.
Is there anyone that can tell me what composition and designation 2S-O aluminium falls in please?

Yours in faith
GadMark

 

[aviat]

2S-O would be annealed 1100 now. Designed were changed in 1954. Info from ANC-5.