Which one to purchase?

That stamp only tells us the strength of the balance, not its identity. I'm having a little trouble reading it; can you confirm what it says? Check the other balance from the window if you are uncertain. Otherwise, the identity of this balance is based off the fittings on either end. This is either a Series 380 or Series 385 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. The video on this page goes into further detail as to how to narrow down the balance you need. 

Comparison of the S378, S380 and S385 balance shoes.