Series 351 I believe?

Hey Mike. If it's not the S351-29, then it's the S390-29. The difference with these two balances is the depth of the bottom shoe, which is the measurement from 'heel-to-toe' and perpendicular to where the metal channel ends. See the illustration below to get what I mean. Compare both with your old balance to determine what matches best. In either case for a new stamp option go with 2820.

Comparison of the S351 and S390 balance shoes.