replacement window balancer

Thanks for posting. Because the top fitting has wings, or side tabs, but the bottom fitting does not, this balance's configuration reminds me of our Series 395. So long as the bottom nylon shoe has a depth, or 'heel-to-toe' measurement, of 1-1/8", I believe this Series is what you're looking for. See the diagram below for reference.

Close up view of the S395 balance shoe.

Next you need to choose the length. Now, you purport the metal channel length is 20". This would be unusual for a balance stamped 2790--the length of the metal channel is usually 1" longer than those first two digits, so I'd expect a metal channel length of 28". Please confirm. If it's 28", you'll need an S395-28. If it's in fact 20" long, it's possible this balance was cut down to size by whoever originally installed it so it would fit your window, and you'd need the S395-28. In either case however, you're going to need to obtain the exact weight of the window sash. 

Finally, you have to choose a stamp option, which designates the strength of the balance. Now, the stamp 2790 is from a rating system we do not use; furthermore, if your balance is indeed a different length than 28", the stamp printed on the balance would be inaccurate to go by to begin with. So what you will need to do in either case is weigh your window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under the product description on the balance shop page. Let me know if you have further questions. 

You're welcome! I was happy to help, of course. Please feel free to reach out if you had any further questions.