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In 1980 the 22nd modern Olympic Games were held in Moscow. To promote the event, all camera manufacturers issued one or more commemorative models of their 1979 or 1980 cameras. This compliancy is so uniform that it seems to indicate they were commissioned by the government or the Plan Bureau.
It is noteworthy that commemorative Zenit-E's with a 1978 datestamp are quite common, and that Princelle even documents the existence of commemorative E's from 1977! For pictures of a 1978 model, see the Zenit-E page.
Types of Moscow-80 silk screenings include the standard one with the official logo (to the right), and the one often seen on KMZ Zenits, with "MOSKVA 80" in a latin font that makes the K almost look like an H. Why people sell these straight-facedly as "Moshva" cameras is above me.
Because of the huge number of Moscow '80 variants, you can't say they're either very rare or very valuable. They're worth more than a standard model, but not substantially more. Their real value is in their high "Soviet memorabilia" coefficient.
These cameras are known to have existed as Moscow '80 models: