Series 770 Channel Balance 27A?

Thanks for posting. First of all, I recommend obtaining the exact length of the metal channel to ensure this is in fact a S770-27. If it is, also make sure your balance has the same bottom attachment (the 15-043 clip) and attaches to the top of the window jamb view a screw. Let me know if anything differs. Finally, obtain the exact weight of the window sash. If the weight falls within the range supported by the 27B (11 through 17 lbs), then that stamp will work fine for you. If it's a different weight, let me know. 

I'm not entirely sure why we don't carry the lightest stamp option for this specific balance, but I'm willing to bet it's because it simply isn't used a lot. For this stamp option we would be talking a weight range that spans less than 10 lbs; you don't often see window sashes that light for windows large enough to require 27" balances. Clearly this stamp also has some overlap with the 27B weight range, hence why you can use that option instead. Basically, it's probably a spring calibration that is almost never needed.