My stockpile of over 120 original wooden "Z" knobs (plus there are hundreds of more original knobs on my actual radios).  These knobs fit sets from 1936-1938.  Zenith introduced the embossed "Z" or "lightning bolt" knobs with their 1936 model line-up. Unfortunately, in 1939 Zenith started using plastic knobs on new 1939 models.  Some 1939 radios did continue to use the wooden knobs and were primarily electronically feature-modified chassis in carry-over cabinets from the 1938 line-up.  Such models include:   9-S-365, 6-S-362, 6-S-361, 6-S-341, 6-S-330, 7-J-259, 6-J-257, 5-J-255, 5-J-247, 7-J-232, 6-J-230, 5-F-251, 5-F-233, 4-F-227, 5-X-274, 5-X-248, 5-X-230.  Some sets, 200-series farm radios, were completely unchanged from the previous year and still carried the 1938 model number.  Many people have speculated about the manufacturing process Zenith used to emboss the wooden knobs. They apparently used a metal stamping die with hot water or steam to soften the wood and create the raised designs on the knobs.  Sorry, none of these knobs are for sale.  Original "Z" knobs do show up for auction on Ebay or amazing reproductions are available from www.antiqueradioknobs.com.