Needing to determine what window channel balance I need

Thank you for sharing your photos with us. Depending on the 'depth' of your bottom balance shoe will determine whether either the S351-30 and S390-30 channel balance would work. The 'depth' measurement of the bottom balance shoe for the Series 351 is 1" and for the Series 390 is 1-1/8". Compare your pieces with ours and see which of the two is a better match. Since your stamping does not correlate to any of ours, you will need to weigh your sash on a scale, and refer to the sash weight chart listed on the product page.