Evgeny, first of all may I implore you to use much restraint in cleaning this extremely important sabre!
This weapon corresponds to sabres discussed some years ago in studies of the 'L-guard' hilt which apparantly may have evolved in early 17th century Eastern Europe from Italian hilts with similar feature in 15th into 16th centuries and quite probably other sources.
The site of the Battle of Beretschko (1651) in the Ukraine was excavated in the 1970s and documented in an article in 'Muzealnictwo Wojskowe' (Tome 5, 1992) : "Battle of Beretschko in 1651 in Light of Historical and Archaeological Sources" by Igor Svieschnokov.
During the Chmielnicki Uprising in 1651, Ukrainian Cossacks clashed with Polish forces, and this particular site among several thousand artifacts were some 32 sabres, many similar but three with this distinctive L guard. In Wagner ("Cut and Thrust Weapons" p.212) a Polish sabre with this feature is shown, and it should be noted that the thumb ring is seen on the 'Beretschko' examples. It appears that the hilt was in use in Poland sometime in the early 17th century.
The mention of these examples from this battle simply illustrates the form in use c.1651 in Eastern Europe, there are similar Austrian examples from later in the 17th century.
What is key here is the deeply stamped cartouche near the langet on the blade, which appears to carry possible Islamic script or characters. It is known that allied with the Ukrainian Cossacks were some Tatars and Turks.
In discussion of Ottoman blades of these times it does seem that stamping of blade with deep circular cartouche was known in certain cases on one side only, possibly arsenal ? It is indeed possible that an Ottoman blade might have been in a Polish sabre of the time, or that similar hilts may have been in use by the Cossacks with Turkish blade.
In discussions I have been told that Cossacks used various types of swords which included Polish types as well as Eastern (incl. Ottoman) forms, but there a few actual examples of these Ukrainian swords (except the Berestschko find). Most are the result of repairs and refurbishing which accounts for certain amalgamations rather than congruent patterns.
With this it is compelling to suggest that this may be an L guard of Eastern European form of the first half 17th century with possible associations to the Ukrainian Cossacks.
With that I would include the distinct possibility that this stamped cartouche could be a tamga, family marking used by Lithuanian Tatars, though that is a remote option. Tamgas are rarely seen on weapons to the best of my knowledge, but the Lithuanians were a large component among the Zaporozhian Cossacks, and I thought it worthy of note.
I note that you have posted this sword on another forum as well. While anticipating response there, please feel free to share any of this information with them so that they might add it to thier notes.
All very best regards,
Jim