Anderson Narrowline Sash Spring
A handy person from Littleton MA says:
May 22, 2016
The number is either B31 or 331 followed by 1/19 1135. Only other number on it is 11600-17. Assumption is the number is 331 which aligns with one of your replacement sash springs.
Paul from SWISCO responded:
May 24, 2016
Based on your stamp of "331," I recommend our 24-331 Andersen sash balance.