I need a pair of these

If the metal channel is 34" long, then I think you need an S795-34 with a stamp option of 34C. You are also going to need the 15-195B Open Cam T-Lock Shoe, sold separately. The video below shows how to swap this shoe out with the one that comes by default with this balance (our 15-195). It is an easy process. 

Before ordering, make sure your balance attaches to the window jamb with a screw-on clip, rather than a hook. If it's a hook, show us a picture of the hole in the window track where the hook attaches to before proceeding.