Window channel balance
Thanks for posting. I believe this might be either an S380-30 or S385-30 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel-to-toe', so compare both options with your old hardware to see what matches best. See the diagram below for reference. If neither seem right, please let me know and give detail as to why. Make sure that the pulley system from which the cord with the clip runs from comes out of the 'guts' of the metal channel, rather than the bottom shoe itself. If its the shoe, please show us.
You then have to choose a stamp option, which reflects the strength of the balance. Your old stamp of 29-9 is from an outmoded system we do not use. To know what stamp to select, 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 balance's shop page.