sigma 150mm - 500mm DG OS HSM
 

I've been looking at a few options of getting a Sigma 150mm - 500mm lens and have been trying to work out if there are older and newer versions of said lens that all share the same name? i.e. that as they improve the quality of the lens they don't give it any distinguishing reference in the title of the product so it's harder to tell if you are buying an older model or not?
can anyone help me out on this?
I have read some lens reviews that suggest this happens.

I am looking at two options at the moment and the writeup for each is quite different and while all the main points are the same the only difference I can see on the advertisements is that one weighs in at 1,910 g and the other at 1,780g.

so now I'm considering they would try to make it as light as possible so possibly the lighter version would be the better of the two or perhaps the extra weight is due to better newer components that are unavoidably heaver?

does anyone have any knowledge of this at all?

as far as i can see there is only one version of this lens and its only dpreview that has it heavier so the reasons could be? one weighed it with the tripod collar on.

ok thanks for that.