27 inch sash with clips ,marked 26 4 I 0 caldwell

Thanks for sending in your photos. I believe you have either our S380-27 balance or our S385-27 balance.
The difference between the two lies in the shoe. The series 385 balance has the 15-004 shoe which is slightly larger from front to back, as illustrated. 

Comparison of the S380 and S385 balance shoes.

If you measure your shoe as it is pictured here, you should be able to determine which one you need. Once you have determined the correct balance series, you'll want to select spring strength option 2640.