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serial number chart tells you the age:
http://www.billswelderrepair.com/WELDER_AGE.htmlI have addressed this same subject in previous posts:
Since code numbers span a 2 to 3 year period it is hard to note that. The serial number of the machine will let you know its age. And the field coils in 73 and 74 were aluminum.
When aluminum windings are talked about, know that it is only speaking of the field and series shunt coils (in 73-74). Because since the 1940's the interpole coil sets have all been aluminum. I have pics of generator housings from 47 to 97, I can post them and you try and tell the difference in the age and type windings.