Thanks for posting. I am sure we can help, but that stamp is not an identifier. It just tells us the strength of the balance. Can you send us photos of the nylon attachments on both ends of the metal channel? The combination of attachments used determines the Series balance you have. We'll do our best to help from there.
Based on the photo, this certainly could also be a S390-29, just with a different stamp option (2840). Just make sure the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, matches what is shown below. If it is only 1", you'd need the S351-29 instead. Let me know if you have any questions.
That's great! I'm glad they're a match.
Thanks so much for the business and please let us know if we can offer any further assistance.