The only marking is the "27 S"

Hello there. Based on the fittings on either end of the metal channel, this might be an S350-28, but please compare to confirm. You then have to choose a stamp option, which designates the strength of the balance. Your old balance's stamp of 27-S is pretty unusual however. Could you give us the exact weight of the window sash that this balance supports? That exact weight will help us clarify what replacement stamp you need. Many thanks. 

Thanks! In that case, I recommend using the 2750 stamp option. Two balances with this stamp can carry 26 through 30 pounds, which should be perfect for your sash weight.

Please feel free to reach out if you had any other questions.

You're very welcome! And thank you for the business, it means a lot.

Feel free to reach out if you had any other questions, we're always here to assist.