Balancer for Milgard series 1510 single hung non tilt windows.

Let's take this one balance at a time. Your 2030 balance corresponds to a 21" balance with a 2030 stamp option. Your 2640 balance is a 27" balance with a 2640 stamp option. Both of them are either Series 380 or Series 385, but I cannot tell which. They are likely both the same Series of balance, but you should verify it. Check the Series on BOTH balances before you order. To tell the difference, measure both shoes as shown below. A 1-3/32" shoe means you need Series 385, but 1" means you need Series 380.

Yep, that sounds like Series 385. Once you have the Series, you only need to know the length and stamp option for each balance, which we determined above. You can verify the lengths and options (21"/2030 and 27"/2640) by measuring the metal channel (metal only) and weighing your window sash. The window sash weight can be compared to the chart on the balance webpage.