serial number. Just PM hifiman. They replied to me the next day.
Serial numbers are under the pads and on the warranty card.
From Reddit:
End of 2017 - Sundara released at $500
Beginning of 2019 - Sundara dropped to $350
End of 2019 - stealth revision changing the pads and dust filter to increase durability
September 2021 -
first reports of the softer 4XX cable, but many people still got them with the old cable even in 2022, not clear if it was regional or maybe based on stock
June 2022 -
adjusted process to drop the impedance to 32Ω - nobody measured them yet to confirm any audible changes but Hifiman assures it sounds the same. Looks like everyone is finally getting them with the new cable
July 2022 -
new packaging with included table stand
The only confirmed change in the sound happened at the end of 2019 when they changed the pads (or should I say the glue in the pads that slightly changed their shape). Not clear if there are any audible changes since the impedance drop. Probably not.
The differences between old and 2021 Sundara were not big on my test fixture: