Window side balances DH not flip out

Thanks for your photos. Based on what I see here, you have either our Series 351 or Series 390, depending on the depth of the bottom shoe. That's the only difference between them. I recommend measuring your shoe and referring to our dimensions to confirm the best match.

Comparison of the S351 and S390 balance shoes.

You don't want to measure at an angle, no. What you're looking for is the overall depth, back to front. The goal is to ensure that your replacement balance has a shoe that will fit comfortably in the track while braced against the sash, rather than being too tight.