From my experience, the patent search won't help much...
Sometimes the boxes have printing dates on them...I've got a set of Champions that have 4-41 printed on the boxes.
Another good method is to look at advertising, for instance, Champion advertising shows plugs with brass tops from 1935-41;
The brass top evidently went away by 1942 as evidenced in this ad;
In 1948, advertising shows a different insulator (different ribbing);
