Need matching 2540 window balance

Hello there. That stamp is not actually an identifier; it instead tells us the strength of the balance. Channel balances are identified by the nylon attachments on both ends, which you have helpfully provided pictures of. This is either an S380-26 or S385-26. The only difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel' to 'toe'. See the diagram below for reference. Compare both balances with your old hardware to see what matches best. In either case, for a new stamp option choose 2540, just like your old balance. Let me know if you have any questions.

Comparison of the S378, S380 and S385 balance shoes.