My spiral balance looks different
Also my old spiral balance looks different. I have a spring over part of the rod and the whole thing was inside a plastic tube which has disingrated. I measure the length from the center of the top mounting hole (your drawing shows measuring from the extreme top end which is another 1/4 inch in length)to the tip of the blue thing and I have 27 and 1/4 inches. Do I order 27" or 28"?
Our replacement spiral balances have a metal covering versus your old balance's plastic covering. The plastic covers are not as strong and tend to disintegrate over time (as yours did).
I want to check your balance diameter. I find a lot of users who have the plastic tube balances do not measure the diameter with the plastic tube on the internal spring. Remeasure your balance and see if it is either 3/8" or 9/16"
The 3/8" diameter tube is our Series 900 and the more popular 9/16" tube is our Series 600.
Since your length measurement is really close to 27", I would recommend you go with the S900-27 (but again as I mentioned earlier in this post, recheck the diameter of the tube because you could need the S600-27).
I would also like to check which strength balance you will need (the color of the balance tip represents the different strengths). Since our metal balances are stronger than your old plastic balances I want to be sure your new balances will not be too strong. To check the strength, all I need you to do is to remove the sash from the window and weigh it on something as simple as a bathroom scale. Respond back here with the weight of your sash and I will let you know which color tip option you will need. See this guide on how to remove a sash from a tilt window.