22" (metal part) "21A" stamped on side

Thanks for your photos. Based on what I see here, you have either our S351-22 or S390-22, 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.

Furthermore, your stamp of "21A" should correspond to our 2110 option, but you may want to weigh your sash on a scale to confirm that it falls within the given weight range.

Comparison of the S351 and S390 balance shoes.