Theory: Luthor's warsuit was always suposed to have a glass dome but the super powers toy forgot to include it and eveyone copied the toy.

So lex's warsuit first shows up in action comics 544 but is shown with a clear dome rather than lacking any face protection. As far as I can tell the first time it didnt have the helmet was the super powers toyline that likely droped the helmet due to parts count or the fragility of a small clear peice.

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Let me know if I missed any earlier appearances of the warsuit not having a helmet?