Hi Alexander,
I also have a Leningrad like yours, serial No. 006508, also with the old speeds, smooth top of the winding knob, and other features the same as yours. Although mine has a box and instructions, there was not a passport to be able to date the camera.
I believe that Leningrad cameras with this kind of early number are also early in the regular production run and date to 1956, with the serial number representing the actual production (ie. ... your camera will be the 9,298th camera made).
Probably after 10,000 cameras were made, the numbers started to have the year date before them (1e. ... No. 560009 ... for 1956, as illustrated on page 318 of the New "1200 Cameras..." book.
Your camera looks very nice and maybe some would consider it to be a pre-series since the number begins with "00..." however, I think the different style of serial number probably just coincides to when the factory decided (or was told by the government) to use the year as the first two digits.
Here is a link to photos of my camera so you can compare ...
http://www.nightphoto.com/leningrad.htmlRegards, Bill