Yeah the Swing. They also have the Groove Key GIP.
Groove Key
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I think the work on these along with the modular design foundation they have allows them to do other prototypes quickly as well.
Just me speculating but i think we may see other protos in this form factor coming up over the next year. Also watch for more on the Groove Key coming up soonish.
As far as why mini-keys, forgive me for stating the obvious. It’s a size and cost intermediate between a full sized keyboard synth and their Minis, that works well enough for people who are not extreme keyboard players.
As Behringer has been moving toward, i would not be surprised with other Oscar products from them in other form factors.