if you are certain it is SK5 steel ..... buy it !!!! It will be old old old. Are there blade serrations .... if so, it is not SK5, I say that with good certainty. As best I know, all SK5's were non-serrated.
with that said, I am a huge huge fan of SOG's SK5 steel, they really got it right in terms of temper and all that metallurgical stuff that contributes to edge taking and edge holding etc etc.
and with that, even if is not SK5, and you can get a good price, do the deal. You can't have enough SOG's ......
Cheers,
Carl