Juhani,
if you look from above you can see the piece where the top of the rewind knob was screwed in. If you take a closer look you can see that it is flattened on two sides (hard to describe). You take two smalls tongs (pliers?), with one you hold the fork, with the other you grip the flattened parts and screw it loose (normal way, left to loose it). It could be that there is a small set screw on the top part, I don't remember. Then the fork comes off.
After that you screw off the part on top with the two slits, then the part with the GOST numbers comes off. You can see two holes in the lowest part (the part you can turn for the measurement). If you turn it right you can see screws under the holes. Screw them off and the bottom part of the lightmeter comes off. I hope I have remembered all correctly :-)
Ulrich
http://fotos.cconin.de