Need help ordering 5/8" x 27" #2640 B.S.1

Hello there. Both the top and bottom attachments lack wings, or 'side tabs', correct? See the image below for reference as to what I mean.

Diagram on how to choose between the 15-001, 15-002, 15-004, 15-005, and 15-029 channel balance shoes.

If they indeed both lack wings, then you have either an S351-27 or S390-27. The only difference between these two balances is the depth of that bottom attachment, or shoe. The depth can be considered the measurement from heel to toe. See the second illustration below for reference.

Comparison of the S351 and S390 balance shoes.

Compare both balances with your old hardware to see what matches best. In either case, for a new stamp code choose 2640.