There are a couple different versions of the 55-300mm. The SMC DA L is the cheapest and I think it has no markings and comes without a lens hood - it could only be found bundled with a camera. The SMC DA was sold on its own, and has distance scales and green ring. The HD DA is the latest version, with the latest coatings and WR. The there lenses all have the same glass, but the HD version has "improved" lens coatings, so it might perform a little better than the other two. WR can also be important, because if you have a WR camera and want to use it in poor weather, you need to put a WR lens on it as well.
More info here:
Review: HD Pentax-DA 55-300mm F4-5.8 ED WR - Introduction | PentaxForums.com Reviews 55-300 Lenses - Pentax Lens Reviews & Lens Database