The black one is a Bob Navitimer strap, and is the closest to the genuine available. Its not perfect, but is good enough to fool most. The stitching is nice and heavy, and the leather is thick. The genuine is slightly thicker at the lugs and tapers towards the buckle. The stitching on the gen is slightly more inboard.
http://www.waccex.de/catalog/index.php?cPath=149_154
The tan strap is one that I bought locally. The stitching is not as rugged, and the leather its not as thick. I will at some point be picking up the Bob version of this strap because I really like the color combination!