This was the third in a series of four articles by E. Buony. It described a double-fixed hub, with different cog sizes on each side. To change your ratios you removed the rear wheel, reversed it and reinserted it. The slots in the rear dropouts were long enough to allow you to accommodate the difference in chain length.
You still see the odd, elderly, bearded, eccentric or fashionably goateed, urban-style-warrior riding double fixed today, and you certainly saw thousands of bikes sporting double fixed hubs at the York Rally as recently (!) as the 1970s.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)