Need to ID old track slider

I recently purchased several of the the 39-253RB and 39-253LB dual encore operators to replace my old operators on my windows. I also purchased a handful of the 39-529  because I found that the old track sliders almost 20 years old and were in bad shape (often cracking or breaking when I was trying to remove or install the new operator). However, the 39-529 track slider doesn't seem to fit right in the track of my window. It's almost like it doesn't have along enough leg on it to properly retain it in the track. Pictures attached show the dimensions of my old slider and me holding up the new 39-529 slider in the track. Do I have the right track slider? Am I doing something wrong? Thanks in advance for any help/guidance!