Not sure what brand of windows

Thanks for writing in, and apologies for the delay in responding. While this stamp code is one of the stamp options we list for our S751-22 Channel Balance, stamp codes are not generally used to determine the identity of the balance you have. Stamp codes designate the calibration of the spring inside the balance, and these rating systems have changed over the years. The only way to reliably say what balance you need is to see pictures of the attachments at either end of the balance. Confirming the exact length of the metal channel, not including those attachments, would also be helpful. From there, we'll try and narrow down what you need. Many thanks.

The stamping that appears on the balance indicates the spring strength of the balance and how much weight it can withstand based on the weight of the sash. Take a look at the sash weight charts listed on the Series 770 balance store page to get more details.