I have 2 Mixtes & from what I've researched, most brands that offered Mixtes did not sell them as their highest end models. Not to say they're bad, but Mixte frames typically weren't outfitted with the highest end components of the time. For commuting, and general purpose riding they're fine bikes and, at least my two, are light.
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Heliocoptors - could you take some photos of the drive side of the bike & post? Tell us derailleurs, shift levers, brakes, rims/hubs, whether the bike is lugged or not, wheel size (27" likely, but may be 700c), handlebars drop or straight. This will help us understand.