Block and Tackle Balancer Stamping says 2601 BSI LE

Thanks for posting. I agree, this does look like an S770-26 at first glance. Just make sure this balance mounts to the top of the window track with a screw, which is how the standard Series 770 is installed. As far as the stamp option goes, 2601 is definitely unusual. I advise you weigh the window sash that these balances support. See if that exact weight matches one of the weight ranges listed alongside each stamp option listed under the product description on the S770-26 shop page. If it falls into one of those weight ranges, that's the stamp option you should select. If the weight falls outside all options, please let me know and we'll proceed from there.