well, I managed to give a closer look.
disk carriers are 100% identical and there's no P/N or S/N anywhere on it, and inside it's only a standard Serial ATA HDD with nothing else added.
It's not like cards and transceivers that have custom electronics and may justify the newer -C variants.
They're both standard Seagate EXOS HDDs with original firmware, no custom mods.
Also same size, same disks, same heads and same sectori size, very similar data rates.
Only difference is 26000 comes now with a newer Seagate Exos 7E8 variant ST1000NM000A from 2019.
Last 23000 RMAd disks are Seagate Exos 7E2 variant ST1000NM0008 from 2017
Initial 23000 disks are Exos 7E8 ST1000NM0055 from 2016
So there is model variation over time also on the same platform, as very common for server HDDs also.