That is a coil spring Low Wall with a single set trigger.  And a Niedner barrel.  Be still my heart!  And the takedown feature (only works with the coil spring action) is rather rare.

The single set trigger works by pushing forward on the trigger (from the back, of course) until you feel a click.  Then you have a very light trigger pull.  Using it as a standard trigger tends to wear the parts.  You can get replacement parts, for $$, but you need to find someone with a special set of skills to work on the SST.

The whole rig is worth some serious $$.  I'm curious about the caliber.  Maybe an ordinary Hornet, maybe not.  Lots of variants back when this was made.  A careful chamber cast would tell you a lot.

It may be a .223" groove-diameter barrel.  The bullets are still around, but probably special order. 

Early on, but about the time that takedown actions were first made, Niedner used base-band bullets.  That requires a different setup and special loading tools & techniques.  If it turns out to be intended for base-band bullets, the shooter-owner is about to enter another universe.

Michael Petrov has passed away, but I think someone on the ASSRA forum has Niedner's record books.  I think it is possible to learn a lot about this rifle.  I'm going to send the ASSRA guys over here to look at this one.