An on-line friend - introducing Mikhail Kretchmar, a hunter, traveller, writer, big-game guide and gun nut - discovered it in Vladivostok and posted pictures at Russian gun community.
Looks like a pre-WW1 Merkel combination gun. The top barrel is 16-gause smoothbore. The bottom barrel used to be .577 BPE, originally, but later converted to 28-gauge Paradox: the grooves were drilled out in most of the barrel to make it a "shotgun" under the USSR/Russian gun legislation; and there's an insert in the chamber to use 28-gauge brass shells. The original cartridge seems to have equal to 24-bore brass shell dimensions at the rim. Now let's see if I can insert some pics...
... oops, something doesn't work (must be the java script disfunction). Anyway, you can see the pics here:
http://community.livejournal.com/ru_guns/665103.html?view=8658447#t8658447 Sorry about the inconvenience.
Two things that don't look common in this gun:
First, the top fastener - it looks like a Koersten, but, yet, it doesn't look like the Koersten we know from the later Merkel O/Us. What is it? This question comes from Mr. Kretchmar.
Second, the choice of the rifle barrel - a .577 BPE doesn't seem the German gunmaker's top choice, and a combination of "ultramodern" O/U design and "obsolete" black-powder cartridge somehow don't fit. This is my question.
So - what are your most welcome opinions about this gun?
P.S. sorry about possible misspellings.