3010 stamp

Thanks for reaching out! Your balances look like they could be either the S351-31 or the S390-31 with a 3010 stamp. 

To determine which of these is the series you'll need, confirm the dimension for the depth of the bottom shoe. It should either be 1" or 1-1/8". Let me know if it doesn't match either measurement.

Comparison of the S351 and S390 balance shoes.

You are measuring the top guide and not the bottom shoe. The bottom shoe is the end that has the hook as well as the wheel and pulley system. Please reference the diagram included above in my initial response.

Once you've confirmed the shoe measurement, you'll be able to select the balance you need using the diagram provided. The shoes are labeled according to the balances they belong to in the diagram.

You either have the S351-31 or the S390-31 with a 3010 stamp.

It is likely you'll need the 3030 balance for the window you removed that balance from. 

If you are unsure of the window weight, you'll need to obtain the exact weight of your window and select the stamp option based on the weight ranges it can carry.

The part number for the balance is the S351-31. The stamp does not determine the series needed for the window, the shoe does.

The 3010 stamp is rated to carry 7 to 12lbs, the 3030 stamp is rated to carry 16 to 23lbs. This is a huge difference and could be a contributing factor as to the frequency you are needing to replace the balances.

I understand that they are all the same sized window, however, size does not account for the weight of the glass. The stamps indicate the weight range that the balances should be able to carry. Given that you have different stamps in different windows, it is advised to obtain the exact weight of the window.