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It looks like the first appearance of the SplitLevel House in a Sears catalog was in 1933, with these two examples. The catalog text noted that these houses employed “a new principle of construction which is a big money saver” and that their “flexible plans” provided “the maximum living area at the lowest cost consistent with good construction” -- something that Sears called a "stepped-up-level house."
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Here's the actual printed advertisement for one of those two houses: the 1933 Colonial Revival-style "Concord" Split Level house, with the little cross-section diagram showing the "stepped-up levels" of this new house. But that's not all: