Thanks for your photos. Based on what I see here, you have either our S351-13 or S390-13, depending on the depth of the bottom shoe. That's the only difference between them. I recommend measuring your shoe and referring to our dimensions to confirm the best match.
Tom is out of the office this week but I would be happy to assist you. Could you confirm the exact length of your original balance's metal channel, not including the end attachments? This type of balance with a 1210 stamp should measure 13" long, but some handymen choose to cut down a longer balance instead of installing balances that are the correct length. I suspect that may be the case where someone cut a 13" balance down to 12" for your window. Just let us know if this is the case and we can go from there.
If the original balance channels measure 12" long but the rest of the specifications match the S351 balances that you purchased, it sounds like you would need the S351-12 instead. You will most likely need 1110 strength options but I would recommend weighing your sash to confirm that it weighs between 3 and 6 lbs. Our return policy can be found here to send back the 13" balances that you already purchased.
