I thought it was also, but try as I might by pausing with my DVR, I could never see it good enough to decide for sure if it was a RSK-1 or a plain 551.
The character of Bob "Cool Breeze" Brown, did have a Griptilian 550 last season, featured a lot in Episode 8 "Natural Selection", but the new one definitely has thumb studs.
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In omnia paratus