Sorry if that's known but I've only learned today that on eBay you can get a board which tries to look like stm32f4discovery while it's only a copy
on eBay they list it as "Stm32f4 Discovery Stm32f407" (spelled with a space)
note how audio connector is green
note lack of ST logo
note how all LEDs are blue
and serial-over-USB does not work on this example
and it costs above $19.90 the real one costs in the USA!