AFAIK, the only difference between the 16800 and 168000 is in the case steel - the 168000 being the first model to incorporate the newer, more corrosion resistant 904L steel. Thus there's no other visual cues to tell them apart just by looking. Perhaps Rolex or someone on a Rolex forum would be able to tell you which model you have based on the serial number of your watch.
I do notice in your photo that the "168000" doesn't look to be perfectly centered in the space between the lugs in the way that the "ORIG ROLEX DESIGN" engraving appears to be. The "168000" is slightly offset to the right. However, without that last "0", a marking of "16800" would be centered as well as the other text. This is total speculation on my part and hopefully I'm wrong, but this could be an indication that 16800 was, in fact, the original model number and that the last 0 was added on later.