Since nobody else answered your question, yes, that's an SG 550. The easy way to tell is the bipod; the 550 has a 20" barrel and longer hand guard with the recesses for the folding bipod. The SG 551 has a 16" barrel, shorter hand guard and no bipod.
He's got some sort of competition diopter rear sight on there; looks different than the Swiss military issue part.
