EDIT: After typing the below comment, I see that by "upper" James is speaking of the top carburetor... not the top orifice. However, at any rate, your question is answered (I think).
James... Top carb 55D is a high speed jet, part #320921... It is not the top idle jet.
The bottom carburetor high speed jet is 56D, part #318684.
These high speed jets are located horizontally in the bottom center of the float chamber.