This is a monocular sight not a focusing monocular. It is a ON-M59 Made by the Yugoslavian Company Zrak Optics.
It is signed ON-M59, ZRAK, 26226, Br 24126. 3-45-395-8/A on front & 1-45-395-0/A on the mount.
Its leather case with strap is signed RB 11470 & ON 42895, so the sight & case may have been swapped about.
This features an integrated internal rangefinder reticle with elevation/height adjustment knob which works (the black one).
It has a dovetail mount, accordion rubber eyecup, yellow solar filter lens (fits in objective sight) using tool which is included.
The bottom (green painted) screw is a disassembly screw to adjust prisms, which are perfectly aligned.
Magnification is 3.8X (non adjustable) weight 1.1 lbs approx, heavy duty metal housing in Yugoslavian military green paint.
The sight was used on tripods, the Zastava M53 Machine gun, the Zastava RB M57 Anti-Tank Rocket/Grenade Launcher.
Typically used by Special Forces during the violent breakup of Yugoslavia and the subsequent Balkan Wars of the 1990's.
Overall I think the unit is in great shape please see video & pictures.
Quite modern and quite available, but also very cool.
the next bit is AI
The Monocular ON-M59 Optical Sight from Zrak Optics Yugoslavia is a rare and sought-after piece of military equipment from the 1960's-90's. This field gear, issued to special forces in Yugoslavia, embodies the militaria theme of the era. Made in Slovenia, this optical sight was used during the conflicts of that time, making it a valuable collectable for enthusiasts of military history. Its design and origins reflect the unique strategies and technologies of artillery operations during this period, adding to its historical significance.