During my first flea market visit in 2012 I found something interesting, which I haven't found being mentioned in any database in the web so far:
A standard Activision cartridge with a red label and the text "Activision PROTOTYPE" printed on it.
Someone attached a white sticker to the top of the label and wrote "River Raid" on it.
As I don't have a regular River Raid cartridge I can't tell if it differs from it.
I attached a picture of the cartridge and two screenshots showing the copyright and company logo.
What do you think about it? It looks pretty authentic.