Block & Tackle Balance

Trying to sort out the right combination of clips and length for our windows. There stamped with 28C, but measure 29 inches long (not including the plastic parts at each end). Looks like a 1/2" channel though.

Here's some pictures of both ends which I hope you can use to tell me the right number for the plastic end pieces.

Thanks...
Mark K

and here's the last picture....