Thank you. These work perfectly and they are higher quality than the originals. The springs are more flexible so the sashes go up and down more easily than ever and they seem like they will last longer. We are very satisfied and will be purchasing more soon to replace the originals, more of which have now broken.
Found this post. I too have Bonneville windows from about the same year. Mine are slightly smaller: lower sash weighs 22.5 lbs, length of the channel balance is 28". Unlike the one in the photo, there is no ID or model number stamped on mine--took some time to determine they were Bonneville, but I have verified they are. All details in the photos above match perfectly.
Can you sell me something that is in my size?
much thanks
Paul