Thanks for posting. See our 24-531 Andersen Sash Balance. This is meant to replace balance boxes that include the stamp code '531', or '5#31'.
Now it looks like your old balance lacks the 'tab' on side that our newer Andersen balances feature. This means you will need to do a little extra work to install the replacement. You will need to cut the header of the window with a wood chisel to make room for the replacement balance's tab.
