The smart box is the same device that @schnapps has mentioned (NowTV 4K Box).
Regarding bonus streams;
When a Bonus Stream is available, you'll find the option at the bottom of your NOW TV app homepage.
That maybe the case when using the NowTV Freeview Smart Box with the on going delay pushing out the new look NowTV home page screen layout.
Though NowTV have assured Freeview Smart box owners that the new look screen layout will eventually be added to their boxes.
And the NowTV Help articles points to that all NowTV boxes will have Bonus Sports Streams (none of the help articles i have read or seen mentions Bonus Streams will not be supported on NowTV Freeview Smart boxes).
Hopefully Simon J will officially confirm this.
This information which i have copied & pasted below was taken from one of the NowTV Help articles.
Got a Sky Sports Month Pass, and one of the devices listed below? You can access extra channels on selected sports events using Bonus Streams, including additional EFL matches and F1 Race Control.
You’ll find all available Bonus Streams at the bottom of your NOW TV app homepage. Don't worry if you can't see any Bonus Streams right now – they'll appear automatically by the time the event starts.
Keep in mindYou can’t access Bonus Streams with a Sky Sports Day or Week Pass. NOW TV Boost features, including full HD and 50 frames per second, aren't available on Bonus Streams.
We'll be making Bonus Streams available on the NOW TV Stick and Boxes (except the white NOW TV Box) later this year, and via the NOW TV Player and supported smartphones in 2021.
I have no idea about when the new look UI will be pushed out to NowTV Freeview Smart box owners, where you will have to keep pestering Simon-J The NowTV Community Manager.
I have seen the new look UI on my NowTV Freeview Smart box briefly during the Trial, but it wasn't much difference to the new UI on the other NowTV boxes and Stick with the addition of the live TV Freeview channels on the TV Guide in the top left hand corner of the Home Screen (virtually identical to what you see now on your NowTV Freeview Smart box).
I did have a major problem with my NowTV Freeview box during the Trial, where i lost all Freeview channels for about 3 months (even though the box detected 170 channels when during a rescan or factory reset) so i couldn't use the Freeview channels and gave up on the box.
The Freeview channels suddenly appeared a few months later on the box after the trial finished, where every week i kept checking to see if the Freeview channels reappeared on my box.
I haven't got enough device slots on my account to keep using my NowTV Freeview box anymore, where i have other devices that i prioritise on my registered device list.
Though i could get my NowTV Freeview Smart box connected up to see if the Bonus Streams are added, but my gut feeling without the new look NowTV UI home screen page they will not be there.
I can only assume there is some sort of hardware conflict with the inbuilt TV tuner with the new NowTV UI.
I doubt NowTV will divulge any information, apart from that they are working behind the scenes to get the new UI rolled out ASAP for NowTV Freeview Smart box owners.