Block and tackle balancer

Thanks for writing in. This looks like it might be either an S351-28 or S390-28. The only difference between these two channel balances is the depth of the bottom nylon shoe or the 'heel-to-toe' measurement. See the diagram below for reference. Measure the depth of your old balance to see what matches best.

Comparison of the S351 and S390 balance shoes.

You then have to choose a stamp option, which designates the strength of the balance. Your old balance's stamp of 27-9 is from an outmoded rating system. To know what stamp to pick, weigh the window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under the product description of either the S351-28 or S390-28 shop page.