This is a well known ancient scan doubler.
Cypress CM-397It suffers from only being a 16-bit scandoubler (i.e. not good for AGA) and being external can suffer from Digital->Analog->Digital->Analog conversion problems (ringing, halos etc).
It can only output 60Hz so PAL scrolling jerks around a lot.
I do not recommend them, way too expensive for what they are. A guy called Roy Justace was selling functionally identical products for $49 (although I think he's put the price upto $69)