Kinco side balance single sash; the metal portion is 29" in length ; need a R and L :

Hi Bob. This looks like either an S351-29 or S390-29 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, as shown in the image below. Compare both with your old hardware to see what matches best. For a new stamp option choose 2820. Note that these balances are non-handed, meaning they will work on either side of the sash. 

Comparison of the S351 and S390 balance shoes.