Window balance and lock questions

A home owner from USA says:
I love your site, very good and helpful. Two questions:
I have around 15 windows to work with.

1- My balance measures 1/2 inch, I think I should goes with series 600, but I like to know can I use series 900 instead? Because smaller balance can hide in the track little better. However, will my existing screw be too big?

2-Do you sell the thing (metal) on the top of the windows to lock or hold it on place so it can't slide down? (see the red box area, not all windows has it, look like a horizontal S shape)

Thank you
User submitted image of their window hardware.
Profile picture of Paul
Paul from SWISCO responded:
Thanks for posting. Can you confirm the diameter of your outer tube? It should be either 3/8" or 5/8". Make sure to measure it as carefully as possible. If you are 100% sure it's 1/2", please show me a picture of it. There is a very special type of balance that is 1/2" in diameter, but it has to be custom made. As for the Series 600 and Series 900 balances, they are not interchangeable.

I'm also not sure what the other part is you're looking for. I can't make out anything in your picture. Can you remove the part in question from the window and show me a clearer picture of it?
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