Yes, it looks like that may be the one but it is worth calling OneGrill and ask them if their model would fit, the Weber rotisserie has chrome plated steel spit and forks, OneGrill's is all SS...and $20 cheaper!
I don't know the Summit B specs but OneGrill produces an excellent product, (stainless steel) I bought one for my Gensis Gold C and I'm very pleased with the quality and performance, they make a model for the s420 model...
That is precisely why I bought the OneGrill rotisserie, it is all SS and the motor is weather proof and rated for 40 lbs...although I don't think the rod would rotate properly with such an extreme load. The motor is massive and practically silent. Besides I will be able to use the rotisserie...
I see, you mean the one with the "lightly damaged" bowl and lid? If I had the money I would go for the one with the damaged table, a phone call to Weber CS and you could get a replacement table for free.
GGross soak the knobs and the ignition button in a solution of 50% hot water and 50% bleach for a couple of hours, rinse and brush with a soft brush (I use an old tooth brush) they will look like new!