Could the "BG07 DE" be an UN european export marking?
I am pretty sure that is what that is. My Steyr M95 Police Carbine has the same marking on it.
To the OP unfortunately there isn't too much detailed information on the Steyr M95 in English so a lot of the markings your going to have a tough time figuring out. The good news is you have a Police Carbine (based on the front sight). These were usually 'Stutzen' (short rifles not carbines as the carbines didn't have bayonet lugs if you look online you can find out the specific type it would have been based on the type of sling swivels) that were modified for police work. This involved removing the bottom sling swivels (there should be a filled spot on the rear bottom of the stock, and if you look at your middle barrel band you will see there is also a washer or broken off piece of metal on the bottom were a metal sling swivel would have been), and replacing the front sight with a taller one. The good news about it is that since they replace the front sight with a taller one you can actually hit the target at 100m with point of aim (unlike most Steyr M95s where you have to aim significantly lower to hit accurately at 100m).
Under the 'S' if you look closely you might be able to identify what year it was made (it will have a WN and a number like '17) though sometimes the number was erased/faded to much to read. I think the 'K' might stand for Kaiser which is King or Emperor in German but this is only a guess might have meant royal approval or some such thing.
For history of your rifle it has had a interesting one. It starts with whenever it was made in Austria (back when Austria-Hungary were together). As a Stutzen of some sort it would have been issued to mountain troops and the like. After WWI it could have ended up in many different places (but likely stayed in Austria), and eventually it was converted to 8x56r by Austria. Then it was likely given to Bulgaria by the Nazis after Germany took over Austria just before WWII. Bulgaria either used this rifle through out WWII as it was (issued to some troops) or it converted it to the Police Carbine standard right away (not much is known on when exactly the conversions were done, personally I like to believe after WWII as it would have made sense for the type of people using these rifles). The Police Carbine was issued to some real nasty types, much like the Gustapo or KGB of Bulgaria post-war. It was intended to be used for 'Internal Security' purposes. Then after all that it likely was placed in storage sometime in the 60s or 70s (some were sold off to Mozambique) and was imported recently (hence the DEC 07 stamping on the barrel).
Personally I love the Steyr M95 action I just think it is very novel. Neat (though expensive) caliber, uses Mannlicher clips, and a fair bit of history. With this you have one you can actually enjoy shooting (reasonably accurate at 100m and I haven't tried farther) it as you don't have to aim a good 3 feet under the target to hit center of mass (no joke, I couldn't figure out why my old long rifle wasn't hitting the target until I aimed at the bottom of it and saw bullet holes in the wooden top part of the frame).