It is a Stevens Model 35. The caliber should be marked on the barrel - normally .22LR. The missing parts are the locking bolt, spring and button; all of the parts which constitute the locking mechanism - the button screws onto the threaded shank of the locking bolt shaft. The pistol cannot be safely fired without the locking mechanism. Yours is an earlier pistol, with a cast trigger guard - later pistols had a sheet metal guard. Unfortunately, I can't provide a source for the missing parts, though you can find a complete list of them, with drawings, at the Numrich/GPC website, I believe. These are really nice little pistols, and yours appears to be in good shape overall, and well worth restoring, if the parts can be found. Alternatively, a really capable gunsmith should be able to make the parts you need, though he would need to fully understand their design and function. Good luck!
PRD1 - mhb - MIke
