which Channel Balances should I get?

This certainly looks like either an S380-28 or S385-28. The only difference between these two balances is the 'depth' of the bottom nylon shoe, so compare both with your old hardware to see what matches best. You will need a stamp option of 2740.

Allow me to clarify. The stamp option indicates the calibration of the spring inside the balance, or essentially the weight range that the balance is meant to support. A stamp option of 2740 is for a window sash in the weight range of 2740 19 through 26 lbs. We can deduce this is the stamp option you need due to your balance's original stamp code of 27-4 (which is equivalent). Hope this helps.