18" Caldwell

Definitely sounds like an 18" channel balance with a 1740 stamp option. You verified the length already, so you still need to verify the option, and determine the Series.

Start by identifying the Series based on the channel balance guide, which will have you compare your plastic attachments to our selection.

Once you determine the Series, verify your option by weighing the sliding window sash and comparing to the chart on the item webpage. Check the chart to tell what the weight range for a 1740 option is, and make sure your sash falls within the range. If it doesn't, select the option that works for your weight.

It's S380-18 or S385-18. Series 350 is definitely not it. Measure your bottom shoe to determine which you need. Series 385 uses the plastic shoe shown below, measuring 1-3/32", and the Series 380 uses a 1" shoe instead.