The letter ‘Z’ which can be detected in the seen on the firmware versions. With that, Samsung are well-known that they use the letter for test firmware. Same thing happened before with the Nougat test firmware update for the Galaxy S7 and S7 Edge last year.
Unfortunately, no further information is known about the Oreo update for the Galaxy S8 and S8 Plus. Neither the release date is known, nor in what markets it will be made available in. With the Oreo update, Google will make changes in their Project Treble to help the manufacturers release their major software upgrades quicker than before. Also, we can only suggest that the Oreo update for the devices will come at the end of this year.
Also, it is very possible that the Galaxy Note 8 will also receive the Oreo update for itself. There is still no doubt that the S8 will be the first device in the line to receive the update, even though there might be other devices which are older (like the Galaxy S7 for example) to receive the update.