What Model number do I need ?

I am afraid that stamp number only tells us the strength of the balance. To know its identity, please remove it and send us pictures of the nylon fittings on either end. We'll do our best to help from there. Many thanks. 

This is possibly either our S351-28 or S390-28, though the bottom fitting is hard to make out on your old balance and might be damaged. Compare with an undamaged version to see if these balances do indeed look correct. The only difference between these two balance Series is the depth of the bottom nylon shoe, or what you can think of as the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the illustration below that shows the difference. In either case you would need a stamp option of 2740. Let me know if you have further questions.

Comparison of the S351 and S390 balance shoes.