You have a machine made by Standard Mfg - the back screw is the stop motion - it tightens against the ring that is right behind it. this one works a bit different - that secondary ring is what the belt goes on, so the flywheel will still turn the needle, but if you watch, the belt drive will stay still - it disengages the belt drive from the flywheel - not the flywheel from the machine.
not sure if I can help on the tension. My VS is a top tension model, and my two Standard rotarys have a slightly different tension setup.
It looks most like the Standard "Paragon" class, though not identical. At least getting shuttles and bobbins won't be super hard - there are plenty of them out there that fit Standard VS machines.
http://needlebar.org/cm/displayimage..._display_media