Identification for 2 different windows

Thanks for posting. This appears to be either our S351-29 or S390-29. The only difference between these two channel balances is the depth of the bottom nylon shoe, or the measurement from heel to toe. See the picture below for reference. Compare both options with your old hardware to see what matches best.

Comparison of the S351 and S390 balance shoes.

You'll then have to choose a stamp option, which denotes the strength of the balance. The stamp 2890 is from a rating system we do not use. To know what stamp to select, weigh the window sash that these balances support. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under the product description of the S351-29 or S390-29 shop page. 

Finally, you also show balances stamped 2820--I highly advise removing one of these to confirm it too is eithe an S351-29 or S390-29 before proceeding. Let me know if you have further questions.