This lens was made for the half-frame Belarussian “Chaika” (”Чайка” = “Seagull”) half-frame cameras.  The 2nd and 3rd models of the Chaika were designed to have removable lenses.  No one is sure why this was done -the first Chaika model had a fixed, non-detachable lens-, but I think it was to allow the lens to [...]