Coil Balances, need several

I have several windows that need coil balances. The photos are from one of them. I tried a set of S140 5lb coil balances, but the bar on the bottom of the window doesn't fit into the window carrier part of the balance. Is there a different model that will fit the bulb on the end of that bar on the existing windows, or is the S140 the right part, but I need to get a replacement bottom bar? Other than the bar, the S140 fit in my window frame (though it was tight). Also, after I ordered, I got the window out and weighed it at approximately 13.5 pounds, does that mean I should use 5, 6, or 7lb balances?