Although soviet camera industry produced a lot of commemorative cameras, only very few Fed's were among them. That's why an anniversary Feds becoming very scarce nowadays. To commemorate the "Great October - 50" Fed issued two cameras: Fed-3b and Fed-4b with special engravings. The 3b is less common.
Although mass-market Fed cameras have no date prefix in their serial numbers, the commemorative Fed-3b has special serials both on camera body and Industar-61 lens. In my case camera body has serials #6724326 and the lens # 6763205.
This camera was also released, without engraving, in a special commemoration box. You can see an example of this at
http://cccp.webstar.nl/fed_3l50y,htm
these cameras also had the prefix 67 to the serial number.