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Demonstrators, based on a list of occupations for 1910 & 1920 census, worked in sales. They may have been models, perfume girls, or something similar in a department store.
Salesmen and saleswomen:
790 Sales agents
791 Salesmen and saleswomen--stores
795 Wholesale dealers, importers, and exporters
Thank you, Susannah! That's a great resource.