After the Germans unveiled their new M35 Helmet design, Bulgaria began designing their own based off the M35. Although different, these helmets bear a striking resemblance to the German helmets they were inspired by. During WW2, these helmets saw action on the Eastern Front, and were an Olive Drab color with the Bulgarian Tri-Colored Decal.
After the War came to an end, Bulgaria continued to use these helmets, and in 1951, refurbished the shells, in the new lighter Olive Drab color, and replaced the Liner with a WW2 German style liner.
Three shell designs were made, the Model A, B and C. Model A's will feature the rolled rim, and are generally heavier. Model B's will have the same liner system as the Model A, but with a raw, unrolled edge, and the Model C would have a new, contact-free suspension bracket installed, and had a Raw edge as well. The Model C's are the most common, and thus they will be cheaper. The model A and B are much less common, and will command a higher premium.
Bulgaria still issues this helmet to some reserve units and other paramilitary units. That's a pretty impressive service life. You'll definitely want to grab one of these for your collection!
A Couple of notes: The Pictured Helmet is a Model A with the Post War 1950's+ liner. The helmets I have contain all different types of liners as some were not updated as drastically. Please expect much more wear use, and soldier graffiti. Some will have decals or remnants of decals. These may also have some of the wool from the liner band flaking off, and resembling a rat's nest. I assure you, it isn't, and it pretty normal of items this old.