Thanks for posting. It looks like you might have either a Series 380 or Series 385 Channel Balance. The difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel' to 'toe'. See the diagram below for reference. Measure the depth of your old balance's shoe to determine what Series you need. Let me know if neither seems right.
To choose the balance length, measure the length of the metal channel. Do not include the end attachments in that measurement. Finally you have to choose a stamp option, which designates the strength of the balance. Look for a stamp on the metal channel that indicates this. If there is none, or you are uncertain, weigh the window sash that these balances support. You can use that exact weight to choose the correct stamp option. Let me know if you have any questions.
