Yes it is a Sigma 70-300mm D DL lens...the mount looks to be Nikon...
http://www.shutterbug.com/content/sigma-af-70-300mm-f4-56-dl-macro
Some info about the lens...
This is what the Nikon mount looks like..so you can verify it...
View attachment 2491
The lens I want to say is an early 2000's lens...like 2002-2005 production...may be older...there were two different verions of this lens...your's and a II...the II had different rubber pattern...look up the lenses on Google...
Seems like you either got a good copy or a bad copy there was not inbetween...
Hope this helps....
Tony C. :z04_cowboy: