Alenco 1/2" Wide Channel Balances

Hello, I am stuck as to which channel balance I need. The non-tilt Alenco windows that I have in my house were installed 32 years ago. The channel balance I need to replace is 1/2 inch (9/16ths?) wide and the metal portion is 28" in length. It also has "2730" and a "J" stamped on its shaft. The problem I am having is I get to the portion of your parts indicator and I can't tell if the S351 or S390 is what I need, because the pics for both look as close to the one I have out of all the choices. I am assuming the difference is the dimensions of the end pieces.

Please let me know what further info you need to help me figure this out. Thank you for your time. MB

Hello Sir. Thank you for your prompt response and the info I needed on what balance to order.

Additionally, I have another quandary. The Top Sash Guide that I have I cannot seem to find the right one amongst what your company offers. Instead of trying to write down the dimensions I have attached a pic of a crude drawing, with associated measurements, to this thread. I believe I am right on with all of them, except for the longest side. It is either 2" or just a hair under, definitely not longer than 2. Also the bottom hole is where the one screw attaches it to the frame.

Hopefully you can help. Thank you, MB