Originally Posted by Zeriman

No production model has a round yellow cap like that, but Saris did make some with yellow capped versions for Slipstream, for some rental units, and I believe samples for vendors. The SL+ and the 2.4+ (what Saris calls an ANT+ upgraded SL 2.4) look idential from the outside, so to tell the difference you need to take the freehub off. The axle on the SL+ is thicker and the freehub on the SL+ pops off without tools, while on the the 2.4+ you need to loosen a bolt to get it off.