21 12 window balancer

There is no way to answer your question without seeing some photos showing us your balance. The numbers in the balance only indicate the spring strength of the balance and in no way helps with the identification of the part. Once we see the part, we can see which of our Series to suggest. 

While you gather everything for us, you may also want to take a moment and view our selection of different balances we offer here.

Thank you for the photos. Your balance closely resembles both our S351-22 and S390-22. The only difference between these two balances is the depth measurement of the bottom shoe. The depth of the bottom shoe is 1" and for the Series 390 is 1-1/8". Measure the depth of your bottom shoe, like in the diagram below, to determine which balance is the best match. Since your stamping does not correspond with any of our stampings, you will need weigh your sash on a scale and refer to the sash weight chart listed on the store page. 

Comparison of the S351 and S390 balance shoes.

I think there's a good chance the strength of the balance you purchases (2120) was too weak. I'm actually not sure what your old stamp (2112) represents, that's fairly uncommon. Before you return anything, I recommend weighing your sash on a scale to get its exact weight. Then, refer to the chart on the S351-22 and let me know which option covers that weight best. We'll work from there.

If it's under 37 pounds then I would go with the 2170 option. The 2175 might be a little too strong in that situation.

That's my opinion, though. If your weight is pretty much 37 pounds on the nose then you could theoretically go with either the 2170 or 2175 option, since both cover that weight.