Window Balance 26-4

Hello there. I think you're probably right, as the only other possibility is the Series 351. This is a very similar balance, with just a slightly less deep bottom shoe. The Series 351 has a shoe depth, or 'heel-to-toe' measurement, of 1", whereas the Series 390's shoe is 1-1/8" deep. Based off the photos your balance's shoe looks closer to the deeper end of the spectrum, but measuring is the best way to be certain. Compare to the diagram below to be certain. Whether you have an S351-27 or S390-27, you will choose the same stamp option: 2640. Let me know if you have further questions.

Comparison of the S351 and S390 balance shoes.