Having just received a couple of 1590s myself this week, the firmware I was offered was what @_Val_ mentioned in his post.
Also, it sounds like you just bought the appliance, which I don’t believe includes support.
The NGTP/NGTX package for the SMB appliances includes support, which entitles you access to software downloads.
All that said, I don’t believe the firmware offered via the on-device update menu is tied to a support contract (but could be wrong).
You might try making a backup of your configuration and factory resetting the device to see if it offers you the newer firmware.