BSI 21-C

Thanks for posting. This looks like a stamp for a channel balance. I am confident we can help, however there are many kinds of channel balances, and that stamp is not an identifier; it just tells us the strength of the balance. We'll need you to remove the old balance and show us clear photos of the nylon attachments at both ends of the metal channel. See the videos listed on this page. We'll do our best to help from there.

Thanks for getting back to me. Looks like you have either the S351-22 or S390-22 Channel Balance. The difference between these two balances is the 'depth' of the bottom shoe. See below. Measure your old balance's shoe depth to see what matches best. For a stamp option choose 2130.

Comparison of the S351 and S390 balance shoes.