Replacment springs

I purchased a house that has double hung windows that have broken springs. The plastic piece at the to is actually what is broken but I cant find the plastic piece by itself. My springs also have a white collar that cover the spring so you do not see it. The one I have out measures 27.5 inches and looks to be 3/8 in diameter. Based your recommendations of replacing both I would like to order 10. the smaller windows have a white tp where the larger have a black tip.

Also I have looked all over my windows and cannot find any name brand or markings.

Thanks Tom. the plastic pice I was referring to was the piece at the top that attached to the top screw. I see with your design its all metal and will not break like mine have. with regard to the outer tube I do not see them on your website to order with my springs. A couple of mine were very brittle and broke into pieces when the spring broke. Where can I get the outer tubes?

Brian